[coderdude]

Those comments could be used to make automated spam filters less effective. Spammers could post comments that would normally be labeled as spam, but do not contain any URLs. Over time a spam filter would have a harder time distinguishing between cut-and-dry spam and real comments (assuming the admin is marking those spam comments as ham).

Also, I'm a fan of not allowing brand new accounts to post URLs in their comments. It's a no-brainer.