I wouldn't like that either. I think it's ok the way it is. People sometimes leave, but it's also much easier to get new people. DPS leaves, it practically doesn't matter, you get a new one right away. And in exchange for having a couple of people leave every now and then, you also don't have to stick around for the whole dungeon. Seems fair.
And sometimes people just have to leave because something happens. What if someone disconnects and you kick him? Would suck to not get your loot then.