You're right about Sym not being a healer. I have to tell the team to pick a healer all the time when I pick Sym. But her portal and the turrets really make most defense missions a lot easier. Heck, I (and the team, once they catch on to the idea) stop any advance at the first choke point for the rest of the game. It is a bit cheesy and thus boring, I admit. But as long as there are bad snipers in the game, I'll cheese as much as I can. If they don't want to push but rather snipe across the map with Widowmaker/Hanzo, good luck to them ever seeing the first checkpoint.