From the Quick Draw feat description:
Benefit: You can draw a weapon as a free action instead of as a move action. (...) Alchemical items, potions, scrolls, and wands cannot be drawn quickly using this feat.
What is the (balance) rationale behind this? I don't understand why alchemical items (splash weapons, in particular) would be excluded from this feat.
Can I safely house rule it away, or will it come back to haunt me?
Historically, it’s because Paizo decided that Rogues (the ones most likely to take the feat) shouldn’t be so “good” at using such “magical” equipment. See also the ruling that alchemical weapons cannot be used for Sneak Attacks.
It’s more of an “image” concern. There really wasn’t any particular balance concern: Quick Draw was never a particular popular feat, nor was it ever especially powerful. The ability to pull out the appropriate wand, or at lower levels, the appropriate alchemical item, was a useful side-facet to the feat but rarely a reason to take it by itself. Remember, Quick Draw does not give the ability to “quick sheathe” anything, and in fact that’s not an option offered at all in either 3.5 or Pathfinder, so particularly with wands, after you’ve activated it, you need to do something with it before you can use another weapon. Dropping it is a free action, so that’s an option, but it has serious risks since that’s a valuable magical item you’re leaving on the floor.
But Paizo decided that it didn’t think Rogues should be using such items so much, despite the fact that an optimal Rogue will use them quite heavily. That’s the “balance” concern: not that it makes Rogues too powerful, but that it’s too frequently a better option for Rogues than “traditional” roguish weapons for Paizo’s taste.
Unfortunately, I am unable to find the original discussions of the alchemical weapons and Sneak Attack nerf, where this was hashed out.
Note that all of this was done before the Alchemist class existed. While there was no balance problem for the Rogue with respect to Quick Draw and alchemical items (as can easily be seen by the lack of balance problems due to this combination in 3.5), it is possible (though I doubt it) that there could be with the Alchemist. However, keep in mind that neither 3.5 nor Pathfinder is tightly balanced. With or without this nerf, the Alchemist is stronger than the Rogue and weaker than the Wizard. Thus, you can houserule this particular combination, and while it will improve some classes relative to others, it won’t improve them so much that it actually changes which is more powerful than the other.
Personally, this is one of many changes in Pathfinder that I houserule. Several other houserules are also aimed at improving Rogues’ lives (like the ability to Sneak Attack with splash weapons), because they need it.
Reason #1: Because that's what the feat says. You may not want to go with this if it doesn't make sense, though; read on:
We can try to justify this feat's description. Typically, a sword or gun is positioned in such a way as to be drawn easily. Think scabbards and sheaths. Plus, critically, the handle on a sword and design of a sheath is conducive to quickly drawing; that is immensely important in combat.
However, there are a few problems with drawing other items:
Wands, however, are a different story. Reading through this thread seems to indicate most people think it's silly to not Quick Draw wands. I tend to agree. In the case of potions, I would let players Quick Draw them if they're positioned in such a way as to be conducive to Quick Draw. This post (and almost the entire thread) supports this opinion.
My take has always been that this is due to the holster/sheath, not so much the weapon. When someone bucks up for a custom made bandoleer for their potion vials, for instance, I allow the same effect be emulated with other than weapons. -- Mark (source, post #4)
Ultimately, the rules of the game are at the players' and GM's discretion. If you and your players are in agreement that it makes sense to Quick Draw these things given a current situation, then do so. If your party is more rules-oriented, then just read the rules as-is.
In simpler terms, do what makes sense, and take the standard Pathfinder rules with a grain of salt.
Edit: Since you're looking more for a rules balance answer, here are my thoughts: Most of the time, this isn't going to affect anything. As @KRyan points out, this was originally intended to nerf rogues, however, it's not a terribly powerful feat to begin with. If you let it apply to these items, then you will very likely encounter no problems, and if you do encounter one of the edge cases of abuse (like repeatedly drawing to slow down action), then you as the DM can just say it's being taken to an unreasonable level.
As far as I can tell, there's nothing which justifiably breaks the game, even if there are a couple edge cases which do.
I would suggest just creating bandoleer's and other items as DM creations that make the idea more feasible. My general decision as a DM is to give the designers of the game the benefit of the doubt, so for example, I know for a fact all weapons and guns have sheaths, scabbards, and holsters almost always, so they are subject to quickdraw. A wand has a handle and thus should also be subject to quickdraw, but alas in typical lore wands normally do not also come equipped with a handy device similar to a scabbard, or quick access holder.
The solution to this problem? Create equipment that holds the items you wish to quickdraw, and add them to the campaign with the equipment description stating that the use of these items makes the appropriate equipment they apply to subject to quickdraw. This maintains the intent behind the feat, allows for roleplay elements without House Ruling so much as adding content (Which is encouraged, and not entirely the same level as a house rule to over-ride an existing rule). Further its generally just cooler to add any item that adds flavor to the game, players even appreciate this sort of thing as it adds additional elements to their characters.
A rogue who is just dexterous with no rationale or role-play explanation for the remarkable dexterity that obviously should apply to weapons, but not so obviously to potions, benefits from equipping his or herself with quick attach poison and alchemical vial holders on his or her belt, or wand sheaths along the side of her or her leg, or scroll compartments designed for quickdraw which automatically cut the string binding the scroll as a roll and open it as you draw. These sorts of elements add finesse and flavor to the campaign and allow the DM and player to characterize and roleplay the scenes more, making it more fun, and heroic, which should be the goal of the game anyways.
I disagree with the general consensus about Alchemical Splash Weapons not workable with Quick Draw.
While I do conform and agree to the ruling that Alchemical Items by default can't be used with Quick Draw, I don't believe it discounts Alchemical Weapons from being used with Quick Draw (which are mostly but-not-all splash weapons). This is based on how the PRD labels the tables differently between these two categories of Alchemical objects.
When looking at the Ultimate Equipment PRD, Alchemical Tools and Alchemical Remedies appear to be considered Items by looking at the name of the column of those objects. They appear tabled and labeled just like items in the Adventuring Gear and Tools and Skill Kits sections. However, the Alchemical Weapons table is much different; it lists the name of the weapons under columns named Light Melee Weapons, One-Handed Melee Weapons, and Ranged Weapons along with all the other typical weapon details you would find in the Weapons, Eastern Weapons, and Firearms etc. sections such as range, damage dice, critical multiplier, and so on. I interpret this to mean that Alchemical Weapons are distinctly different from Alchemical Items such that the Alchemical Weapons are actually Weapons.
If that is true, then Alchemical Weapons such as Alchemist's Fire and Acid should be allowed benefit from the Quick Draw feat since it intends for Weapons to benefit from it and only excludes Alchemical Items specifically, not Alchemical Items and Weapons.
As a note, I haven't been following Pathfinder for too long and wasn't "around" during the rulings I keep hearing about where Quick Draw was nerfed with the Alchemical Items exclusion. However, in my efforts to research the Paizo discussion forums and other places, I have only found hearsay from individuals-who-are-not-the-designers who infer that the designers intended that Quick Draw change to exclude Alchemical Weapons like Acid Flasks and Alchemist's Fire, but I found no designer quotes cited or sourced, sadly. I would love to see an actual designer source about it though even if to be proved wrong, as I dislike the idea of running games as a GM where I mislead my players by saying something is a house-rule that in actuality isn't or vice versa.
Fortunately, with either interpretation of the intent of the rules (and about a year after the OP question was asked) any character could at very least choose to dip 2 levels into the Rogue Alternate Class "Underground Chemist" to achieve the ability to be "able to retrieve an alchemical item as if drawing a weapon" (and along with that, adding Int bonus to splash weapon damage). As some have inferred, if the rule changes made to Quick Draw were meant to exclude Rogues from making full attacks with splash weapons (functionally) or that the designers didn't like the image of such an idea, I'd be puzzled why such a rogue archetype with such abilities exist as well as the Bomber and Bomber's Discovery rogue talents (unless it was some form of apology). Oddly, I can't find anything on the Alchemist class that allows alchemists to retrieve alchemical items as if drawing a weapon like the Underground Chemist Rogue can (alchemist bombs don't appear to be tagged as alchemical items, mind).
The only thing I can think of that would be realistically broken about this is more an 'intelligent use' case. If quickdraw makes drawing an item a free action, and dropping an item is a free action, then you could do things like grabbing caltrops and dropping them as you retreat a thing. Same goes for alchemist's fire in the paths of the enemy horses, or the Bulette, or any other similar issue (Such as a thunderstone). If at any point players decide to start using the mechanic for things like this, you'll have to correct it in some manner.
The balance reason is that alchemical items are ranged touch attacks that still do damage on a miss. Touch attacks are easier to hit with, plus splash weapons are guaranteed damage. Imagine a feat that let a Wizard get two Scorching Rays per round with no use of wands or Quicken Spell. Against a group of enemies that are close together, the damage can be comparable with alchemical weapons.
