I'm not even mentioning the different roles. Say you run with 2x tank, 3 healer, a rogue, a warrior, a lock, an ele shaman and a mage. If ele falls short you can't just decide to play resto shaman in that roster, and many fights were absolutely not melee friendly either. If you even take out the whole gearing issue, then a mage just stays in his role, just a different spec.
You simply can't use the argument that an ele shaman playing enhancement is the same as a fire mage going arcane. If you do so, you ignore a part of the reality.