Sigvertisements have never bothered me, I think it's cool that people can make a little money for something small and easy.  There can be confusion about what is a sig ad, and what is a forum ad.  This could make theymos's job harder if something illegal/viral is advertised, but as long as those are deleted I don't see a problem.

Maybe provide your own bitcoin forum that doesn't allow advertisements?  I hear that reddit is popular.