Well, if the devs (as they have described) test each job to make sure it's capable of clearing the hardest fights, and if they even provide stone/sea/sky to help you make sure that your skill rotation is good enough to pass the (sometimes irritating) DPS checks, then it would seem that the problem you describe doesn't derive from the jobs themselves, but rather from the party organizers.

If you want to play with players who are intent on finding or using the "optimal" party composition, then you might have to play multiple jobs because that composition will change from battle to battle and from patch to patch. Although I don't share that obsession, I do kind of like the idea of putting together a party where the skills synergize with each other.

Personally I kept playing BLM even when it was "underpowered" in certain patches, simply because... I have fun playing BLM!! I have also noticed that properly executing mechanics is usually the most important thing, because your DPS drops to zero when you're dead.