The MP pool part is true but one of the bigger reasons SMN ended up as healers was because of the Avatar mechanics. Perpetuation costs, Amount of TP they gave mobs in relation to the damage they did. The amount of damage avatars did in comparison to other party members. Even basic avatar stats like there accuracy, when your avatars are eating 9~10 mp every 3 seconds and landing very few hits to show for it. Thats a big reason smn was pigeon holed into healing
While you can, I think you shouldn't be forced to. I'm of the belief that if you're favorite job is dragoon you should be able to complete all content as that class, sure you may need to level other classes to get certain skills / abilities. but you shouldn't be pushed into jobs you don't enjoy playing.
it was one of the big issues in 1.0 with av and cc aside from the drop rates if you wanted to to clear it you needed to stack black mages. or stack monks or stack something else. thats one thing i hope goes bye bye in ARR.
Needs to be a huge diversity in what jobs can do. If i'm on my paladin and not tanking then maybe i could be supporting the other paladin who is. (kind of envisioning some style of the cover system where i would occasionally help the other paladin block an attack or something) maybe simply buff my healing skills a little and help the other guy stay standing. it's a loose concept but diversity is key.
in some fashion whm and warrior had this already in 1.0. whm can pop cleric stance and throw out nukes and a couple of debuffs, warriors can switch between rampage and berserk. hopefully ARR expands on this across the jobs.
but yeah i'm a huge fan of diversity which is one reason i hope to see a ton of job options down the line and all with there own identity. which is partly why i think the armory system was so flawed in 1.0 and hoping its changed in arr.