You aware that you dont want to change the loot-system, Cordie, but only make it more annoying and difficult to use gear for any other purpose but wearing it (may it be for glamour or stats)?
The system you're proposing isnt helping anyone nor is it needed - specially when its obvious that the game intends you to use dungeon gear for other purposes outside of wearing it. With your horrible, horrible idea non-crafters basically dont have a chance to get GC-seals, yet they are crucial to progress within the game - may it be relic, contribution to FC-crafts (like getting material for the airships etc.) or your squadron. And for crafters seals are actually that high in demand that "needing" them is totally valid (but ofc, you cant know that since you're not really a crafter...)
Basically your system isnt benefitting anyone but is screwing a lot of people for no good reason.
Specially when there is a very good system in place to solve the issue: Talk to your teammates and ASK if you can get specific pieces. Sometimes you'll be lucky, sometimes you wont - reminds me about something... mmh... its almost like... rolling a dice on a piece of gear that dropped...
Yes and its a good reason - one that you cant understand when you dont craft. And while its not something that I can ONLY do with the class that can roll need on an item, by choosing a class that correspond well with my desynth-classes I can influence it quite heavly. For example my desynth classes are weaver, ltw and goldsmith - so my best bet is running dungeons as ranged or caster because those classes can roll need on leather and cloth gear.
I dont understand why you're so tied up on the idea that you'd only be allowed to use the piece of gear you got on the class you got it for. I need it. Period. For what reason I need it, is non of your or anyones buisness when the game is designed around needing those dungeon-drops for a lot of things outside of simple gear-progression.