I actually think this is a really fun idea. When i think of the concept of OP in an MMO, i take it to mean that one ability or class is significantly more powerful than others, giving it an unfair advantage when comparing to to other classes abilities. This quarter job concept would give everyone these same 'OP' options, so, it would bump everyone's power equally. And as much of a boost as this would give, this would not suddenly allow every job/sub/quarter combo to start easily soloing alliance level NM's.

Also, the fact that people are arguing over which quarter job would be the 'OP' must have quarter job implies that there really is some choice, and not a specific quarter job that everyone would take, and that by taking that quarter job, it would invalidate that job as a main job.

In fact, i could very much see this quarter job actually give new life to certain main jobs that have been considered weak for a long time, because they don't have access to some tool that another main/sub has. As mentioned above, provoke/utsusemi are considered so vital that they are nearly the only sub options for many main jobs, and are very powerful when combined together as a WAR/NIN or NIN/WAR. Quarter jobs would allow for more main jobs to be able to tank(like some of them used to be able to, or were maybe intended to but were never capable), as those main jobs could /sub/quarter WAR and/or NIN.

this could also give more viable options for subjobs, if you could get certain 'must have' abilities from a quarter job. More mages could sub SMN, since they could quarter SCH for light/dark arts.

It would give many many fun new options and combos, and would only be OP when comparing it to someone who hasn't opened the quarter job option yet via it's quest.