The current loot model isn't perfect, but it works for the most part, when someone leaves, they take a penalty and they immediately get replaced, its selfish but they take the punishment. Plus some people may actually have a reason to leave, it would be bad to punish those people as well.

I never leave early for not getting my drop, I am a "take what I can get" kinda guy, if I can use it and/or its an upgrade for one of my classes, I will take it, saves me the headache of running it constantly for one silly drop. If you feel strongly about it, you can try reporting the people who leave.

The best part is when people ask to be kicked, get denied and then cry about it.