1. So that subsequent readers of their posts can see by their name that they're spammers.
2. It amuses me and others.
Item 1 is irrelevant here. On SI it was the best I could do since it would often take hours to delete all of a spammer's posts and the deletions could only happen at night, leaving the posts out there to potentially cause harm (and set precedent) all day.
A valid question is "Is the amusement factor a good enough reason to do it or is it a bad thing to do?"
I'm in favor of doing it, but I'm not dead-set on it. Someone raised a very valid criticism of it elsewhere, so I'd like to get as much input on this as possible.