Bad DPS can be carried. In a 5 man dungeon group, in an LFR, in a raid proper, there's another DPS that can pick up the slack. There isn't always another tank or healer (whether because there's one, or because they have their own roles, or need sustained target switching). It might be easier to play tank or heals, but if you fuck up, you wipe. Naturally people prefer being carried to wiping (or improving their ability to play, if they even recognise this as an option).