I know I made somewhat of an opposing point a couple posts up, but I'd like to dispute this. Take your classic melee DPS king, the rogue. If you remove backstab, a rogue's DPS becomes more or less identical to a warrior's (up to weapon choice; obviously warriors aren't going for DPS weapons as much, but there is some overlap, e.g. SSOYs). So the question is, just how much DPS does backstab add? Well, I've heard from rogues that at level 50, backstab accounts for OVER HALF of your DPS. In other words, with identical gear, a rogue will do double the damage of a warrior. That's not a small gap.

While I'm not opposed to a group with 2 tanks I would not stay in a group (or let them stay) if a pally/war/sk insisted they were dps and refused to tank.