We have had to turn off signatures on another blog site due to SEO spammers using them to create a link farm. They have robots that create accounts, and insert a bunch of links to client websites. They do not actually post anything, so you have to look for this. The links are picked up by Google and it raises the rankings of the advertised web sites. It also invites the wrath of Google who might just pull any listings for your web site. I had to manually remove about 3,000 of them, so that feature was restricted to being turned on only when a poster had established himself as being valid.