Pretty darn stupid obviously, but especially so when it comes to the dungeon finder tool. Imagine for a moment you are a brain dead DPS who has just waited 20-45 minutes in a dungeon queue, your tank and or healer drop group or disconnect on the first pull, you immediately drop group because you love waiting in queues. This seems to be the reaction I see nine times out of ten.
Tanks and healers I can see dropping at the drop of a hat, they can get right back into another dungeon almost immediately, but DPS have a prohibitively long wait at times and yet still screw themselves and the other DPS in their party nearly every chance they get.
Once you are in a group and zoned in you are at the very top of the queue should you need to replace someone. Even if you need to replace a tank or healer the wait time is going to be tiny in comparison to leaving the group and requeuing as DPS. Even the most impatient person still has the option to use the button on the minimap to zone out of the dungeon, continue whatever you were doing and zone back in when the rest of the group appears.
I'd say about 50% of the groups I join as DPS don't finish the dungeon because the tank and/or healer drop for one reason or the other. It usually doesn't have anything to do with the success of the group either. Anyone can put the group into the queue to fill a hole. Anyone can vote to kick someone that has disconnected or gone afk for an extended period of time. The wait time for grabbing a tank, healer or DPS for a group in progress are all about the same. Yet for some reason the majority of the population seems to prefer to give up on a group despite increasing their total wait time by 15-40 minutes assuming it takes 5 minutes for the dungeon finder to find replacements. It usually happens nearly instantly, but often times people will again drop as soon as they zone in with a group in progress; morons.
So the moral of the rant is that DPS should be trying harder to keep groups going as they have the most time invested. DPS are expendable in the eyes of tanks and healers, but when it comes to a dungeon in progress everyone is nearly equally expendable and DPS should take advantage of that fact when it suits them.
It is kind of ironic that the only people I've seen take advantage of the shorter queue times for dungeons in progress are DPS who exploit it by queuing together as a tank and/or healer and then causing a group to fall apart. They then requeue together as DPS and start at the top of the queue for their dungeon in progress blowing by the rest of the DPS waiting their turn.
It is rather sad to be left in a group of one unable to continue when all it would have taken was one other person to stay in the group and requeue to resume almost immediately with a brand new party.