Fire and Earth Elementals are still the level 70 versions, there is not a level 80 elemental. Basically, they're dead in heroics before the first heal lands on them. Basically you'd be stuck kiting, as an all shaman team without any tanks. Not sure if heroics are doable like that or not. I'd guess, they will eventually be doable, as your gear gets to a point where the instance is trivial, but the trinity of tank + heals + 3x dps will be far superior for heroics.

As far as taking the same toons to PvP, once dual specs are brought in, the druid might be the best tank as they can switch to their boomkin spec for pvp fun. Most tanking/melee types won't be a strong option in BG's, at least compared to running with 5x ranged dps casters.