Know what is useless? Your Class.

[kukurumei]

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.

Depends, though the only job that effectively worthwhile right now is MRD, since a mrd still retains tanky-ness and can put out considerable DPS with the cross class system.

For stages like Titan Ex, you only really need about 40 seconds of an OT tank, which is where cross classes come in.