Earth Elemental totems...

I have only 4 Shamans, and when I go to 5 man instances like Shadow Labs or Steam vaults, not carebear instances that are easy.. I usually can take out the trash very quickly, 1 2 3 4 mobs will die before they can hit me, casters are a joke, i could probably wipe 30 SL casters down before they even got a spell past my grounding totems.. with a LOT of melee in a trash pull, I usually drop one earth elemental totem.

then i have 2 sets of DPS macros, one where all 4 are casting DPS, and one where 3 are casting bolts, and 1 is healing the focus targets target. so you are still doing amazing DPS, while healing an awesome tank.

what usually happens, is the earth elemental tanks all the mobs on him, while one shaman heals him, and the other 3 shamans usually burst down one target at a time, and yes pull aggro off the elemental, but it will die immediately anyway.

so with 5 shamans, you can use this in cycle almost forever if you also substitute a fire elemental and heroism for faster heals for tough caster or light melee groups.

bosses are just as easy, i can use earth elementals for almost every boss i have come across. as long as you have a shaman healing you are golden. if you have one shaman specced resto and set to heal the focus targets target, you can EASILY tank everything in normal instances with decently geared elemental shamans.

this makes them uniquely suited to this situation.

paladin priest DPS combos may be more efficient and less complicated, but wont work in PvP as was said.