If they wanted the armory system to be "you can do any jobs, any time" they would have linked all level, like invoke/scholar (leveling one levels the other). So I agree with Mcshiggs, you can do anything, but you're not "BiS" for everything at anytime, and that's ok.

It's already there, why can't I be i90 with my BLM when i'm i90 with my WHM ? Because I need to farm equipment... It's the same with "farm 10k seals, and swap your stat !" and it's easier !

Though, I would like to be able to sell the Keeper's Hymn for gil, because crafting daily gives a lot of seals ^^