Yesterday, as I was explaining to a client that someone would need to remove spam registrations, the client asked me what difference it made if they are in the system because we can just turn off all privileges that anonymous users don't have.
I was not able to give her a compelling argument of why this clean up is important. The fact that I like to keep everything on a website nice and clean was not a compelling argument for her. What are the security or other risks in not cleaning up the spam users?