I like it, kinda. I think you should be able to equip a job title (like white mage), but still be able to equip abilities from other classes not suited for that job title. However, abilities under that job title (like white mage) receive a significant boost to their effectiveness (like cure, banish, protect, etc.,) while other abilities do not (like slow, blind, etc.,) and perhaps even some abilities suffer penalties (like drain, scourge, etc.,).
That's my two cents.