it doesn't have to, we're only stuck in that mindset because all the existing ones do it. maybe trying to make it not have to is what will make the job different from all the other tanks.
and limited spell list is no big deal, BLM might have a bigger list but the 2.x rotation is more than 50% Fire 1 and the 3.x rotation is still dominated by Fire IV. SMN is supposed to be a casting job and it only has 4 main spells none of which has to do with summoning.