I would say it would depend on the situation you're in. Personally, I like having one in full resto (my 5 shammy group is split out into different specs anyways) but that's all a playstyle thing. Really though, if you are running you're own instances anyways, you are bound to have some healing gear drop, just gather some on one guy and keep it around and swap it out if you are in boss fights/instances that give you trouble.