There is nearly-zero return on investment in Heal
This answer is all about cost–benefit analysis. That is, how much good does Heal do you, compared to what costs you have to pay to get those benefits. For the most part, Heal costs skill points, which are not very valuable, but they are strictly limited and almost every character wishes they had substantially more of them than they do. Which is to say, we don’t necessarily need to get a lot from our skill points, but we definitely want to get something.
Speaking of, you cannot talk about how good anything is in a vacuum; you need to consider how else you could achieve the same thing. In this case, we have to consider magical healing, most notably a wand of cure light wounds. A caster level 1st wand of cure light wounds costs 750 gp, which is an affordable figure for a party of four before even reaching 2nd level. By mid levels, it’s pocket change.
The short answer is, Heal is useless because it cannot replace magical healing, and you can’t ever afford to go without magical healing. Since it has no use, there is no reason to invest in it—it provides no return on that investment. There are only three exceptions:
At 1st level, since you cannot afford a wand of cure light wounds yet.
With Healer’s Hands and Heal’s skill unlock—uniquely, good enough to replace magical healing, though the cost is immense and not worth it in most campaigns.
To torture people, specifically to erode their Will.
Using Heal to heal
The most basic application of the Heal skill is to heal people: treat deadly wounds, cure diseases and poisons, and assist in the body’s natural healing.
Basic usage: only worthwhile at 1st level
The first thing to note is that Pathfinder changes massively between the 1st and 2nd level. Many of your stats literally double, and your wealth grows quite substantially. Where a starting character almost-certainly has no gear of significant value, a 2nd-level character can have some basic masterwork and magical gear.
This matters a whole lot: you cannot afford a wand of cure light wounds at 1st level. That means the Heal skill has lost its primary competition.
It also means that magical healing from class features is fairly limited, and there may be only one person capable of casting even stabilize—so if that person gets hurt, someone needs to try to perform first aid to stop them from bleeding out.
So all-in-all, Heal has some utility at 1st level. A 1st-level character who has it as a class skill might want to invest a single skill point in it, since they get a +4 improvement in the check for one point and they may actually use it.
But at higher levels, the ubiquity and necessity of magical healing triumphs, and hard. Every single party ever absolutely requires access to magical healing; the game is designed around it. Magical healing is so much massively faster that higher-level characters, with far more hp than 1st-level characters, require it to actually be able to continue their adventures. Spending an hour (!) to heal a tiny amount of hp (!), and then having to wait an entire day before you can do it again (!!!) means treating deadly wounds is a waste of time and will not help an adventuring party in any way. And magical diseases and poisons—the ones that are real threats—are often resistant to mundane healing, so it isn’s useful for that either. If such a party finds itself without magical healing, the Heal skill is not a replacement, not even an inferior replacement: the party must have magical healing. You cannot adventure without it.
So magical healing means you will never use the Heal skill. If you have magical healing, there’d be no reason to use Heal, and if you don’t have magical healing, you need to get it.
That means characters will rapidly stop using Heal for anything, ever. As a result, any skill points in Heal after that first one—each point only giving you a +1 on the check—does nothing for you, as you won’t be using the skill in the first place. There’s also no reward for hitting higher DCs—the best you can do is hit DC 25 to treat deadly wounds and add your Wisdom modifier—so once you have a +15 modifier to hit that while taking 10, there is literally nothing to be gained from more Heal ranks. As established, while we don’t necessarily expect much from our skill points, we definitely expect something.
Heal Unchained: nice improvement, but does nothing about the big problems
Pathfinder Unchained allows you to “unlock” skills, Heal included, to allow greater effects. For Heal, this means substantially improving the effects of treating deadly wounds, though even six times as much hp healing still does not result in anything like “substantial” hp healing. The six healed points of ability score damage is more significant, but this is also at 20th level.
Anyway, as written, unlocking either means specializing in that skill as an unchained rogue, or taking the Signature Skill feat. Rogues only get to unlock four skills total throughout their careers, however, while Signature Skill can be taken only once—unlocking only one skill.
So unlocking Heal is a high cost—usually. Rogues rely a lot on their skills and Heal is unlikely to be a top contender, so using 25% of their unlocks on Heal seems quite dubious. Five levels in rogue to get an unlock on a healer is a non-starter: so much magical healing has to be given up for that to be a thing that you will never recover—and yes, you still need magical healing, because treating deadly wounds can still only be performed once per day per creature. And Signature Skill requires taking a feat—itself a large cost—and also locks you out of taking Signature Skill for any other skill. Considering all of the problems with treating deadly wounds (once per day, taking an hour, the numbers still being small), that would be a terrible use of extremely scarce and valuable resources.
On the other hand, Pathfinder Unchained also suggests that skill unlocks could just be opened up to anyone with the requisite skill ranks, as a variant rule. If that is in play, the cost is a lot less... but probably not high enough to justify the skill points. After all, even with it healing more and healing ability score damage, treating deadly wounds still takes forever and still can only be done once a day.
Healer’s Hands: finally competitive with magic, but the cost is high
This feat from Planar Adventures allows you to treat deadly wounds without regarding for a healer’s kit, as a full-round action, and as many times per day as you would like.
Can you say every problem with Heal gone?
With Healer’s Hands, the healing provided by treating deadly wounds is finally competitive with a wand of cure light wounds. That is a massively big deal. You could legitimately go without buying wands of cure light wounds if you have Healer’s Hands. And if you unlock Heal—allowing it to heal ability score damage—you can also skip the pricier wand of lesser restoration.
That still costs probably two feats (or a feat and a major class feature, if you are a rogue). Two feats is monumental in this game. Is eliminating the need for healing wands worth that?
From a strictly monetary perspective, no. The wealth guidelines tracks wealth—the value of the items you actually have in your possession. When you have consumed a wand, it’s worthless—which means you don’t count it as part of your wealth. In short, buying a wand is a temporary expense, not a permanent one. By the rules, you should recoup that expense in the course of your travels, simply because you will use it up and your wealth should recover.
Even if we ignore that—and in extremely abusive situations, the rules for wealth recommend you do, though standard usage of these wands shouldn’t be considered “extremely abusive”—the wand of cure light wounds is still extremely cheap. A feat is worth—minimum—10,000 gp. You can buy 13 wands of cure light wounds for that money and still have some left over. That’s 3,575 hp worth of healing, on average. How quickly are you likely to take 3,575 points of damage that will need healing, to make Healer’s Hands worth it (only considering hp healing for the moment)? There is an extremely good chance that you will never do so. Especially since you can buy wands of cure light wounds as you need them, but you have to “buy” the feat all as one big lump sum.
Now, if you unlock Heal, treating deadly wounds also heals ability score damage—and a wand of lesser restoration is more expensive (4,500 gp). Then again, ability score damage is also far less common. Will you actually go through all 50 charges on that wand? Will you do it twice? Depends a lot on your campaign, but still seems pretty unlikely. And now you’ve used two feats, which is immense.
The other major advantage of Healer’s Hands is it doesn’t run out or require any resources. While wands of cure light wounds or a wand of lesser restoration are probably less costly overall, that doesn’t help you if you need to buy one and you’re nowhere near anyone selling one. Proper preparation can largely alleviate this as a concern, but this is nonetheless a major advantage.
In short: where no one should ever invest in Heal without Healer’s Hands, there is enough return on the investment in Healer’s Hands to make it a valid choice in the right campaign for the right character. In most campaigns, it isn’t quite worth it, but it’s close enough to be argued, and the right campaign could easily tip the needle on that.
Finally, because Healer’s Hands makes treating deadly wounds a full-round action, you could consider doing it in combat. Please don’t—even with the skill unlocked, you are healing less than one attack’s worth of damage at a time. At best, if an ally is bleeding out next to you, it might save their life, but that’s a really unlikely scenario—and even in that case, it’s not actually helpful if the enemy is still there and just undoes it, and can still do other things besides. You are far better off clearing out the danger first. The only healing you should really be using in combat are stabilize—in emergency situations—and heal.
Using Heal to improve magical healing
This isn’t a thing. Magical and mundane healing are completely independent, and having skill in Heal does absolutely nothing for your healing spells.
This is also the important difference between the Heal skill and, say, the example of a mundane weapon. “Mundane” weapon skills are “free” for someone who gains “magical” weapon skills, because of course there is no such thing as “mundane” weapon skills or “magical” weapon skills. Weapon skills are weapon skills, whether the weapon is magical or not. But mundane and magical healing are completely separated, and your skill with one doesn’t affect your skill with the other in any way. You don’t get any benefit at magical healing for having skill at the Heal skill—and having magical healing ability doesn’t make you any better at using Heal.
So unlike a non-magical weapon, which a warrior would use in a pinch because all his feats and class features he intended to use with a magical weapon also work with a non-magical weapon, a magical healer deprived of magic isn’t automatically also good at healing. In order to be good at mundane healing, he has to independently invest resources in the Heal skill. Doing this does not aid him in any way when he can use magic, and as discussed, magical healing is necessary.
Using Heal for diagnosis or anatomical knowledge
This isn’t a thing either. The rules give zero indication that you would use a Heal check for this—everything to do with knowing something about a creature falls under the appropriate Knowledge skill, as the rules are written. A few published adventures give custom uses of the Heal skill for particular set-piece challenges that work this way, but those cannot be generalized outward to the rest of the game, barring a houserule. Technological Guide mentions something about Heal being used to identify drugs and pharmaceuticals, but zero information is given about how that works—and Technological Guide is fairly unusual in a Pathfinder game anyway.
So while being able to diagnose problems would be a useful thing, the Heal skill doesn’t actually cover that. You would need Knowledge, local for most humanoids, nature for animals, and so on.
Using Heal to torture people
Villains: Rebirth gives a new usage of Heal for torturing people. Most of the options are garbage—and torture is not a reliable means of extracting actionable intelligence, which the rules reflect by not making it so in the game. As GM, I would give anyone massive circumstance bonuses on their Bluff check, since the incredible stress of torture make detecting any “stress” one might feel from lying undetectable. I recommend other GMs do the same.
However, one of the things you can do with torture is wear down a target’s Will save. That can be terrifyingly effective, since it can allow you to then use magic on them. There are probably quicker, easier ways to do it, but if you have the luxury of time and a secure, private location to perform the torture, it could be worthwhile for a high-value, high-Will target.
Or, if you want to do this as a large-scale, continuous operation, it scales phenomenally: if your evil tyrant makes a habit of imprisoning people in cells that erode their Will for a month before dealing with them, then even a low-level minion could spend each day casting charm person on several people, with all-but-guaranteed success. You could therefore have a facility capable of dealing with dozens of political prisoners per day, manned by only a few low-level spellcasters. Without eroding their Will, each prisoner would probably require a high-level spellcaster, and/or several wasted spell slots getting the spell to stick, before they could be dealt with, so this approach is very efficient.