They're not mutually exclusively. Take 3.x's BRD and MCH for example. The two shared so much in base foundation, there was very little differentiation despite how (GB/WM excluded) you try to differentiate them down the road. Both use a priority system based on procs, both maintain a self-buff, both maintain a DoT. The only real difference came from their cooldowns and one not being designed for cast times due to the nature of oGCD resets. Compare it to 4.x; both have a rather distinct take on their rotation and gameplay.

I'd honestly rather play a game where every character (or on this case, jobs) plays differently from each other and can be interchangable for the content. It's been said again and again; unless you're absolutely min-maxing, you don't need to bench out certain jobs for content (and honestly, it's also why gear is shared to an extent to begin with).