FFXIII as an example. Sooo I'm not allowed to make my party full of ravagers?
FFXII as an example. You mean I'm not allowed to make everyone get the same licenses and gambits?
FFXI as an example. Its cheating that we're allowed to do summoner burn, whereas EXPing took forever?
FFX as an example. So I'm not allowed to max my sphere grid and make everyone do the same damage?

If you don't get the point I'm making its this:
YOU CAN DO WHATEVER YOU WANT. Having a full party of mages may very well not work out, its up to you. WHY do you care if someone else plays like that, just do you.

The reward for diverisity is supposed to be perfect formation.