"Is it good" is subjective and depends on what your goals are, as well as what resources are available in your setting. But I think that the analysis in your answer is either shortsighted or suitable only for a more lenient campaign setting.
0. Assumption: Foci are desirable
This doesn't properly fit in the list, since a focus is only worthwhile if you can use it to do what you want. It's not hard to imagine a situation where you might need a specific focus, not just a jumble of time/ability calculations against an availability table. So if you need an enchanting focus that will add 4 dice to your enchanting pool, then nothing else will do-- not a focus adding 3 dice, and not one adding 4 to spellcasting.
So I'm assuming that you need more than just "it's technically possible to get this focus, given enough time and no external pressure or problems".
1. Foci are restricted
This is very character- and setting-dependent, but characters without solid credentials (like a real SIN, perhaps a national one) will have a hard time buying restricted items, like foci. The license itself could be expensive, hard-to-get, make you too notice-able to megacorps or antagonists, or be undesirable for other reasons. If you can't or don't want to purchase foci through legitimate channels making them may be your only option.
If you do want to make a legitimate purchase, you can't have a license without an ID. If you have a fake SIN things are even worse-- higher quality SINs are more reliable but much more expensive. Higher-end foci might only be available from a vendor who cares about a valid SIN check, and a rare, restricted item might require a more thorough examination of your SIN, which might in turn require a high quality SIN. Better SINs get expensive quickly, and any SIN check can fail causing it to be burned (losing it along with any licenses associated with it).
Conversely (and your character may or may not care), but when in use foci are not subtle to anyone who can astrally perceive and are tied to your actual identity via your astral signature. A valid-seeming license could potentially save you a lot of headaches, even with a cheaper SIN. Making your foci instead of buying them could help safeguard your investment in each, depending on how much scrutiny you're under when found with a focus versus when trying to buy one.
2. More powerful foci quickly become harder to find, and more expensive
Even if you have an appropriate license (or black market connection), even the most common foci have an availability rating of (Force x 3). So a force-3 spell focus has an availability of 9R, which is already well beyond what you can just assume is available. Searching for what you want at all takes time, and contacts also require investment to be reliably effective. That means you might not be able to find a focus meeting your specifications in a useful time frame, or at all.
And you might well want or need the benefit of a better focus than you can safely buy. Adding 4 dice to spellcasting tests in any situation is a big deal, and might be decisively better in a scenario than adding 3, but availability is 16R(!) at a book-suggested price of 72,000 nuyen. Your GM might make them even more expensive when you purchase them illegitimately, or add other complications in securing something so prized (I probably would). Making a high-quality focus might be easier than buying one.
3. Foci are diverse and may be inconvenient
You're looking for something powerful and low-key for a run next month, and you manage to successfully find almost what you want: a force-4 power focus. But it's bulky and noisy, which make it unsuitable! You're out of luck-- that's 12R, and unless you're really well connected you may not find another option at all before your run. Searching itself takes time. If you can craft your own, you gain options from available formulae or could design your own to suit your exact preferences.
4. Artificing can add value to other investments magic-users may need to make anyways
A magic user will benefit from having a high magic stat, and is therefore likely to have a high magic rating regardless of any other factors. Since that's half of the artificing roll, putting a few points in artificing could be attractive. Additionally, a character using magic may need a magical lodge for some other purpose. That's not an outrageous expense, but a modestly powerful one is still a decent-sized investment for a lot of characters. It wouldn't persuade me at the margins, but if you have to have a lodge anyways it's nice to be able to do more with it.
So if you've already got a high magic rating and a decent-quality lodge, in exchange for taking even one point in artificing you get a way to deal with the other elements in my answer, potentially gain convenient foci you can use, and if a focus comes out weaker than what you wanted you might make some money by selling it.
5. A particular table or campaign setting might make money more valuable than time
Nuyen may be easier or harder to come by, and when it's harder you might need to be more discerning in how you spend it. Some things require you to spend a set amount of money; after paying your living expenses, gear maintenance costs, and other necessary expenses burning most of your purse on a single focus might not be as attractive as other options. Simultaneously, you likely have downtime. The nature of skills and selling goods to vendors means that there are cases where it will be easier to convert your free time into a specific product rather than money.
Successfully crafting an item saves you from paying its purchase price at a cost of your downtime, which you might not be able to otherwise convert into the purchase price of that item (or something equivalently valuable to you). Any nuyen you're currently holding can then be spent on other things, perhaps more vital ones. This won't always be worthwhile, but definitely will be in at least some cases.
6. There are only so many ways to spend your downtime
You can use your downtime for lots of things, but investing it in getting an item or money is likely to either produce something trivial or involve some risk. Making foci is not risk-free, but it might be the most desirable option for a stretch of downtime depending on what other opportunities exist and what skills a player has invested in.
"Good" in this context is only meaningful in relation to specific goals and situations. There are cases where the skill is worthless (like after your character's magic rating drops to 0) and cases where it's very valuable (you want foci but your setting makes them impossible to find and buy). Most cases will fall in between, and while you won't be able to optimize focus production without specializing your whole character around the skill that's true of any specialization. But if you do take artificing, you'll almost certainly be able to get something out of it, situation-optimal or not.