I leveled 1-50 on WAR and PLD in dungeons. It is nice and fun because you get to meet new people and learn to work around all the quirks about other players. If people weren't bad it would be harder for you (as a tank) to get better. If you can tank for horrible players, you can certainly tank for good players.

However on DPS or even healer now it is not a viable way of leveling. Beyond the insane wait time to be partied with a bad tank, there is the dry spot between Brayflox and Stone Vigil that leaves you to level off of Brayflox for ~10 levels. (which ends up being around 4 full runs per level towards the end)

DPS can be bad, Healer can be meh, but if a Tank is bad the party may as well not even begin.

I'd love to level all of my classes in the dungeons, even if it is slower by a large margin, but its just not possible.