Nah, you dont really need constitution.
Thing is, when you add all your skills at summoner, atleast around lvl13 you will have summoning maxed, and no where to go next...
So you want something along with it, and to be honest, choice is entirely yours - my warfare works great with more and more CC and blood incarnate, ranger could be great with inc as meatshield and great versatility in damage types + single target damage, mages are fun with fire incarnate more more elemental mayhem, or you can pick a little bit from every school for mass buffs/heals/support spells.
So going for pure summoner is imo quite suboptimal, as you will have more than enough points to do sidespecialization (and constitution is really garbage!)
