I guess they kept them because its complicated to remove them without reseting levels.

I believe a class's role should end at lv30 when you unlock get to unlock its job, and that job take over the class's place.

You start as a GLA > you play until Lv30 > you have to unlock a job to level any further > you unlock PLD > now you're a PLD.

You know, upgrading from a class to a job as a right of passage for your profession and that's that.