I wouldn't want to see this implemented for tanks. I would like to see it implemented for ALL classes.
You know it is completely understandable that sometimes a dungeon pops, and you are in the middle of a leve or FATE, or otherwise busy, so you withdraw. It happens, no big deal, but then you get these people that; join, leave, join, leave, join, leave, etc... So you are stuck sitting in DF for 10-15 minutes longer, because the one guy it keeps putting you with is withdrawing worse than a room of people at a detox clinic.

I'd say after 2 or 3 times, it should slap a 30 second queue penalty on the person that prevents them from queuing, that is cumulative if they keep doing it. If they withdraw again, it's 60, then 90, 120, etc... Once they accept a duty, the penalty counter resets to 0. Or, if they don't queue up after so long, the penalty will eventually disappear.

It's unfair to blame it on tanks. Join a CT queue, you will see it is everyone, not just tanks.