Tank and Healer would abused it to the max if there is no penalty since they get instant que. They are already doing it even with the penalty, you can expect this features to be abused even more without the penalty. Personally, if you need to leave in case of emergency, 30 minutes is more then enough. Just leave and take the 30 minutes to do what you need to do.

Imagine if there is no penalty for leaving Dungeon or instances.

Tank: Oh I don't like this Dungeon because it is not Brayfox. *Leave*
Tank: DPS sucks! *Leave*
Tank: we are not killing fast enought! *Leave*
Tank: Someone died on pull or first boss. *Leave*

Reque to repeat and just screw everyone over again especially DPS who waited 30 - 1 hours for Dungeon. Seriously no! I'm a Tank, I get instant que, but I still wouldn't want a no Penalty either. It will be very unfair to class who don't get instant que.