Also note that you only see advertisements when you are not logged in, once you register an account and log in you don't see any advertisement ... similar to linuxquestions.org
I think this is a nice compromise ... As ocicat already pointed out, hosting needs to be paid.
But I, for one, am not a particular big fan of advertisement, there's such a thing as banned blindness (
http://www.useit.com/alertbox/banner-blindness.html) and there is this other study which shows that a small number of users are responsible for an overwhelming majority of advertisement clicks (Can't find the research right now).
So internet advertisement doesn't really work (Well, no advertisement really does, but this is a different story).
In addition, I think there are better/more creative ways to covert (part of) the hosting costs...