I think that would reduce the feeling of diversity significantly.
I would however think a single job being able to cast and move or something would be acceptable, which would increase the diversity. On that I think they can add other concepts too like halting a cast midway for some job might not cancel the attack but just launch it off earlier (charge attacks), or channeling attacks, casts within casts (for example you might imagine bard had a channel spell that allowed them to move at the same time, and during that period of playing their instrument you could play 'sets' of music as casts within casts which would either help your team or mess with the enemy - of course this would make the ranger/archer theme part of bard weaker since currently you can use the job gauge for that concept lol, this would be making bard actually play a song

), etc, things like that may add to the different feels to abilities and that would be good.
Also as an aside (on topic of spell use) hopefully SE looks at smart casting from MOBA games (LoL, HotS, DotA)- so we could have cool things like if you tap a spell it just hits your target but if you held it you could manually target and at release it fires off (meaning no multiple key presses but you get more options and streamlined experience at the same time).