shaman don't have the healing/dps boundaries that other characters have.
for example a shadow priest can't cast heals without shifting out of shadown form, a druid moomkin can't cast heals without shifting out of moonkin form, etc.
elemental shaman heals might not be mana efficient but they can at least cast them without having to change out of a dps form.

I have to admit that a 4 or 5 druid team sounds like fun though 8o