I've been looking through the rules about Forgery and I'm confused.

Many things in the Shadowrun world seem very specific, from coded RFID tags to DNA matching and molecular branding. Also, much of it is done digitally, no one really has "papers"anymore. The book says you can use it to counterfeit hard currency, but again no one really uses that anymore and the rules for copying a credstick seem harsh enough as to not really be worth it.

So aside from forgery of things like art, what good is forgery?

You can use Forgery to counterfeit any paper certifications, artwork, and hard currency, yes. With the Edit program you can also attempt to counterfeit electronic currencies (Nuyen, Aztlaner pesos, etc.), falsify data to look like they came from a particular device, or counterfeit other electronic documents (licenses, certificates, etc.).

I have a feeling that it is falling into the locksmith category.
The category of skills that are being obsoleted by technology.

OR

Forgery can still be used for other situations:
Software (or edit) and forgery might be used
to forge a digital signature or object ID;
to convincing stitch a voice sample from scraps;

Digital forgery follows a lot of the same principles
as old fashioned analogue physical forgery.

Unless you deal with obsolete technology.
In the case of the Locksmith skill, you may come across an old-fashioned lock, in the case of Forgery, you may want to forge something that is not a bunch of 1s and 0s.
In a world where everything is digitalized and people are aware of how easy it is to alter digital evidence, a forged piece of handwriting may carry additional weight.
Counterfeit bank notes may be viewed with less suspicion than virtual money from dubious sources.

Of course, such instances will be much rarer than cases where you deal with information sitting on a harddrive somewhere, so the skill is indeed quite limited.
In most, if not all cases where a character of mine could have use for the skill, i'd rather slot a skillsoft or summon a Task Spirit than learning it myself.

Most likely, this would be a skill rather found among my NPCs, be they sweat shop workers producing pirated designer clothes or cunning adepts copying ancient works of art.

However, it's certainly a skill that has potential for downtime moneymaking (not necessarily in a literal sense, though that as well) if applied creatively.
Same goes for Enchanting, Chemistry and other more widespread crafting skills, of course, and these may be more relevant to typical runner actvities.

It boils down to the very basic problem that in a system where all skills (with the exception of knowledge skills) are equally expensive, some may seem overpriced because their use is more limited.

Though this also means that such skills tend to become lost arts, at least among the shadowrunning community and that characters actually highly proficient in them can end up being sought-after specialists.
It's the same as with Escape Artist, Locksmith, Pilot Spacecraft, Exotic Weapon and so on- usually, you won't pick one of those skills, but if you do, it can become a trademark of the character and he will possibly even attract runs no one else qualifies for.
These should naturally pay better than jobs which can be handled by a team with an average skill set.

The problem with this aspect is that one can duplicate said skills easily by the abovementioned means.
But if i choose a character who relies heavily on the use of chips or spirits to become a skill monkey, i tend to be actually more likely to pick up unusual skills i normally won't have the points for, as the versatility of Voodoo or a combination of augmentations and pirated skillsofts relieves a lot of the burden to spend points on "must-have" skills such as Dodge/Gymnastics, Perception, the Influence group or a well-rounded assortment of combat skills.

IIRC, Forgery can be used to forge digital documents as well... Or maybe that's a houserule.

yeah, I thought you could forge anything you had the skill to build. If you can make a credstick, You can forge it, if you make a liscense or a stock portfolio, you can forge it. Of course some of this seems circumvented with the hacking skill. You should need the forgery skill to spoof a lifestyle. but that's my opinion. Also, forgery would be a must have for characters if you could forge a SIN with it. But I think that the fluff says that it takes a bit more than forgery to pull that off.