As in, flames everyone to a crisp, posts as much porn as possible, tries to incite a civil war between a few members that might not be on good terms with each other or the staff and registers hundreds of accounts, some of which stay semi dormant until they strike?
Because that can happen very easily online, especially if you get the ire of someone with a lot of free time and very few morals. Or if your site ends up at war with a troll site/gets raided by 4chan.
Do you avoid the hassle now, or wait until the situation blows up and half the site is now in the middle of it?