It's common knowledge people do things as fast and as efficient possible. If you know beforehand you can't handle that, it literally takes 2 seconds to communicate this to the group. If you don't I'll assume you can handle it and pull in what I assume the group can handle.
Don't act like you don't know how fast a dungeon can be cleared at when you have done it dozens of times. Don't be a pussy, open your mouth and speak up if you know you will perform below the average. (it's nothing to be ashamed off, especially if you are a new player)
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It's a fact.
Average queue time with tanks is 2-10mins tops, dps it's 20-40mins. Meaning at the very least a tank can start his second dungeon before the dps even got in his first. Keep in mind this proces for 2 weeks (let's be generous with the time) and your tank is fully geared while your dps is half way there with some luck.
The tank no longer needs to queue aside from his daily dungeon while the dps still needs to farm twice as long. So in the time it takes to gear up 1 dps, at least 2 tanks are fully geared.
It's something that will never change, you could double the amount of tanks and they will still stop queueing the moment they got all they need from dungeons. Don't forget that raiding guilds only need a few tanks as well, so there is no room for more tanks.
In terms of bad attitude, dps are off worse as they get stuck with toxic attitude after waiting for ages and they can't leave with a serious panelty.
Just to clarify, I rarely had/have any issues in groups as I tend to keep a positive attitude and pull extra in a lightmooded way ("Let's see if we can handle 2 packs?
"). It makes a world of difference in how you present yourself.