The punishment is perfectly reasonable just the way it is. There is too much chance for people to get overly punished for having legitimate reasons to have to leave a dungeon early.

Hell if I'm in a dungeon and one of my dogs start puking on the floor or my wife has an emergency and yells for me to come help her I'm leaving the dungeon. Would be very crappy for me to have a huge punishment for having to leave for a good reason.

I understand it's annoying that people leave, but short of it being a tank (which honestly isn't necessary in low level dungeons, I think i've 3 manned just about all of them with decent players) it's easy to replace and move on.

Seriously though, every dungeon below mid 40's is easy enough that 2 dps and 1 healer can complete them.