Well yeah... FFXI allowed 18 man alliances, but let's get real here.. they were bigger than that . Dynamis originally allowed what? 64 people? It was something like that, been a long time.
I do agree that specializing down too far isn't the answer, for instance you wouldn't want a fight that includes as a mechanic "Use this specific DRK job ability on this thing at this time or you lose." That'd be a mistake, but I'd be very surprised to see that kind of mistake at this point.
Jobs need to cover their role as well as have a niche all their own so that they aren't all just carbon copies of one another with different-looking armor. However, it isn't completely necessary that all jobs be just as good as all other jobs at a particular role. For instance, it's probably ok if BLM does more damage on the fight that requires melee to run around a lot. It's probably ok if having a SMN to hurl a pet at an add helps keep your tank focused on the boss for a few seconds longer while the pet gets eaten. It's probably ok if WHM is a better tank healer than the other healing classes, maybe they aren't as good at multi-target healing.
Those are random examples, but you get the point. It's important for the delta in performance to not be massive, but it's ok for it to exist.