This doesn't make any sense. What if you leave right after the duty starts because the tank is so awful they can't tank anything? What if the reason you left is because you're in a level 47 dungeon and the guy doesn't even have Shield Oath? Or doesn't even have PLD unlocked? There are plenty, PLENTY of things people do in dungeons that I don't appreciate, but I don't think a system to penalize them for being awful would make any sense. People have the right to leave. 30 minutes is already a pretty severe penalty for anyone who is trying to actively play the game. You're just going to have to deal with it.