I run a couple of forums and am curious about the links some people may put in their signature blog. If someone slips in a porn site in a sig block will that negetively affect my site?
IMO it can unless you have enough backlinks and good content to where it doesn't matter too much (just like here on DP where there is a lot of cursing in some of the forums )
Roll with it, but analyse the members tyou allow in. Many people use bots to regiser, and the problem here is that they all get a profile page with a link on it. So kill the shit registrations, and rely on your genuine members to link naturally and your ok.
IMO not a bad thing, but there are reasons others have decided not to display their own sites. (Which has nothing to do with the search engines)