I don't understand...for what reason exactly? I don't think we can 'auto ban' based on vocabulary or some such thing (if that's what you mean), wouldn't feel safe doing that anyway since it'd be sure to get legitimate users eventually.
I don't get it either. I've not seen a 'borderline' case yet, all of the bans I've handed out have been to clear spambots (or people doing a very good impression of a spambot, which is a bannable offence anyway), so I don't see any reason to leave the messages in place. All they'd do is take up space and distract people from real posts made by other members.
Just to show what I'm deleting, here are the four posts made by "efgh508":
efgh508 said:
Very cool article you got! I just added you to my bookmarks!Tags:X Y Z