The reason for splitting is simple. When everybody can be every class in the game, and mix and match abilities as they see fit, there is no reason to take up ability slots in a class for versatility when you can bring your versatility with you. Want to level as as white mage? Make sure you either have a party at hand or level up another caster class first so that you can add some damage spells to your list.

With the current setup, the magic disciplines are a mess and need be redone. We only have two (versus the 6+ for all the other disciplines), and those two are essentially slightly different flavors of the same class.