Lore wise it wouldn't have made sense for Thaum to become a healer. I mean they HAD heals, but it was things like Sacrifice which damaged the caster. A simple swap of their skills would have made more sense. Who knows, maybe in doing so we would have kept the elemental weaknesses and resistances I miss so very much.
Hmm... it's a tough call between these guys and FF12's summons. I mean FF12 summons looked flashy, but they were almost completely pointless to use and a waste of mist. Here in ARR, they don't look imposing but your summoner would suck without it. So at least they're moderately useful.