I wouldn't say that the jobs are useless, events just call for certain types of jobs.
SMN, RNG, and COR can't get the time of day in Abyssea, but they're awesome in Voidwatch.
BLU is useful (for grellow proccing and Charged Whisker farming) in Abyssea, but they're pretty shitty in Voidwatch.
There are some jobs that seem to get the lion's share of usefulness (Black Mage, Warrior, White Mage), but that's generally because they're strategically fundamental. Things like having BLMs proc in Abyssea are a little excessive, but the job would have been as unloved there as the COR/RNG/SMN if they hadn't.
If I were to name a job that has no real strategic uses at the moment, it would be Dragoon. Dragoon's strategic usefulness at the moment hinges on Angon and Ryunohige (aka, bring one for a zerg and Ryunohige DRGs will eat your DD-land cake). Outside of those two things, they kinda have no niche.




