The class/job system is pretty silly right now. Maybe in the future once they really expand the number of jobs and potentially classes things will fall into place...but right now I agree, it's useless.

Particularly with Arcanist. In the early levels it's fine as a DD...but at level 50? You're losing Fester if you don't go as SMN, which is rough. And I doubt Arcanist can be an effective healer at 50.
There is quite sincerely not a reason to go back to that class EVER unless you just miss cute carby's face.