I rerolled one to Resto at 70 when starting Heroics. I boosted/quested all the way to 70 on my 4 Shaman with my Paladin who was already L70 with a T4 level prot gear set. I didn't have any issues doing all the instances on normal with 4x Elemental tossing the occasional heal my way. But this was before MgT was released so I can imagine that one would be a bit difficult to complete without a resto spec healer.