As far as the design perspective, niche gameplay severely limits your choices in content, specially when the bulk of gameplay is dungeons, raids and instanced battles. You can't have situations where you're stacking DRGs or stacking BLMs, or where you want two wars and sit anyone that likes playing PLD. It was that sort of thing that killed endgame for me in FFXI (between niche design, the existence of gearswaps, and Utsusemi being mandatory in close to everything that was "hard"). FFXI is a textbook example of how to not balance classes in a game, and why you should create parity in performance. The 14 jobs that are not the "ace" jobs are effectively wastes of code and space in-game, and that's not something I wish to see in this game.
In a game that you are meant to level more then 1 class, it would make sense that they have strengths and weaknesses. But right now in this game, they can pretty much remove all classes and just make 1 generic tank/healer/dps.

S-E needs to make an incentive to people that makes them want to level up more classes.