Well, yeah some quests require it, but really I can't think of a single fight you wouldn't want your main hero to go along. Being you can level the hero up really high AND he can't perma die makes him the best unit ever. Plus I THINK he only counts as ONE unit instead of 4 like a normal hero unit would need.
But my point above was that, having a second unit that can group heal will be handy. Plus the hero unit only does like 500 HP or something like that, so maybe if the shaman does more we will see more of an edge there.
As far as the dragon slayer army, that sounds pretty handy for pvp for sure. Although really only 1 or 2 is all you need. I mean, a dragon = 10 units and I am pretty sure someone isn't going to just pick 6 dragons, ALTHOUGH it wouldn't be impossible.