I still can't imagine how a tank would fit into the whole thing. If you don't need a tank, why should you bother with the dmg-loss of bringing one?
You could say the same of healers really. It's not like PUGs will have a choice on who they bring, since anyone can queue in any spec. But yeah, if you mean to be running zerg mode, swapping to a DPS stance at least makes some sense. You can probably just do that and not even bother respeccing and get along fine.