When I don't feel like tanking or healing or am on an alt that can't I'll queue for a random dungeon as DPS. The wait is a lot longer, but I'm generally doing something else to pass the time in game like dailies or leveling.
Two times out of three though one of the DPS times out and I am stuck back in the queue for another 20 minutes. Why? The group already had a tank and a healer how hard is it for the system to find us a dime a dozen DPS?
My old theory was we all go back to the bottom of the queue and start over unlike if we had actually zoned it and been in a group together we'd of had first pick. After mentioning that theory many dismissed it immediately saying they didn't notice that happening and insisted that you stay at the top of the queue.
Another theory I have and am leaning toward is that tanks and to a lesser extent healers and dropping and rejoining the queue as soon as a DPS doesn't accept immediately. Then they requeue and get stuck in a group of DPS that was paying attention higher up in the queue leaving my group to wait longer. No skin off a tanks' nose; they get a group immediately, even the terrible ones.
In either case it is DPS hurting other DPS. DPS has the longest queue times of anyone and are actually making their fellow DPS wait longer by going afk in the queue. Obviously if you are afk you won't get a group and will have to queue again so why waste your time and the time of others?
Blizzard should add a mechanic where if you time out on a dungeon ready check you get the 15 minute debuff to discourage such behavior.