So, they're finally at least considering giving us a glamour log?

[Niwashi]

The problem comes from the sheer number of items that could be affected. Every association you add is potentially for every piece of armour in everyone's inventory, armoury chest, retainer, etc.
When they only add one, it's simply switching the map from a fixed item skin to reference a variable skin instead. With each they add, that's another flag for (arguably) minimal benefit.

What you call "minimal benefit" is fixing something that for many players is the biggest problem the glamour system has. (Or in my own view, one of the two biggest problems.) Certainly an important enough one to be worth a couple extra flags per gear item (and yes, that's for all the gear items we're holding, the same as our current glamour).

I'm not quite sure what any of it has to do with unlocking skins for different classes. I'm assuming those rules would either stay as they are now, or that if it changes, that change would be unrelated to this one.

If you have a single glamour slot that sits across all classes, all gear would have to be unlocked or you'd run into issues with say, setting the coat to a Bard-only item and then switching to White Mage. The glamour would no longer be valid.

Which is exactly how it works now. The glamour we apply is across all classes, and if you glamour something that's only available to one of them, then whenever you switch to another class, the glamour doesn't work and the underlying gear is displayed instead. Of course, if SE is making changes in glamour, changing how that works could well be one of them. But whether it is or not would be unrelated to whether they add a couple more available glamours onto the item.

If you did go with the hypothetical 4+ potential skins tied to a piece of gear, the gear sets would still have to be modified to store that flag, so I don't know that it would be significantly easier than just pointing to the skin you want in the first place, but without access to the code or DB schema, I can't really attest to that. :-/

Our gear sets already know what glamour skin we were using when the gear set was stored (as well as what color it was and what materia it had), but the way gear sets work is essentially just saying "These are the items I would like to equip. Are they available?" Then the system checks our armory chest to try and find them. So you could modify the file to say your glamour is an item you've never actually received in the game, but when you try to equip that set, you'd just get the warning that a matching item with that glamour cannot be found. Regardless of what your gear sets say, you can only equip items and glamours that you actually have on hand. That puts the important part of the data with the item itself which SE holds in its database where it's less readily hackable than gear sets that we store.

Without knowing their code base, I don't know what other reasons there are as well, but I'm pretty sure that's one of them. (Though it's one that could be solved if they'd move the storage of gear sets to their own servers.)