in my opinion, if you want pop ups or other ads, then put'em in there. they're your forums to be run how you want them to be run.
will you lose (potential) members because of them? absolutely. its a well-known fact that people hate pop up ads (no need for indepth research on that one!
). but, if you're ok with those "risks," and you still wanna do it anyway, go ahead! no real reason why you couldn't or shouldn't.
if you dont mind my asking, how much
are you making from the pop ups?