Bad Neighborhoods
Google doesn't generally penalize or ban sites based on who links to you (which makes sense since you have little control over that), but you can be penalized based on who you link to. If you're linking to a site that gets banned for pushing-the-envelope, your site could get caught up in a "sweep" of all sites that link to the offending site. Such a linking "infraction" can result in your site getting banned (or penalized) as well.
Is this fair? ...well, perhaps not. Regardless, it means you must pay attention and choose carefully who you link to. Google obviously figures that if everyone is just indiscriminately swapping links it hurts their ability to organize their index in relevant fashion. This, of course, is one reason why link farms and web rings are generally a bad idea. You have very little control over who your page links to ? and it's very easy to inadvertently link into a bad neighborhood.
Maybe people can read that and understand how valuable your tool really can be
