the jobs can greatly limit cutomization and enhance uniqueness without being class specific.
BLM trait, enhanced offensive magic by 15%
BLM skills Xmagic, mana pool, focused spell (increases recast time and dmg)
cross class limits, certain buff skills, and all cures X
just that alone would totally limit and define a role, regardless of weather you were conjurer or thaumaturge
you can do similar things with any job role, and apply it to most classes that would be likely to use it.