i would rather take just shaman healers than priests!!!
they are very good at groupheal and with the right glyphs pretty good at single heal, too. they will beat every other healer with the same equipment and skill if they want to right now in overall heal (recount).
furthermore:
- dispell poison (you DO need this in some situations)
- dispell disease (you DO need this in some situations)
- lots of earthshields and riptide (hots)
- lots of totems, healing stream and mana stream for everyone
- heroism every 5 minutes (you DO need this in some situations - max dmg!)
- dont drop dead as fast as a cloth wearer (last healer standing)
- etc
so what else do you need?