Although I have preventative measures in place to both ban comment spammers and/or prevent comments from auto-posting, I am still scratching my head at the perceived value by the spammers. So:
1) If the comments were auto-posting it is almost certainly devaluing the authoirty of the site - this works against both site owners and the spammers.
2) Again if autoposting or manually approved, the garbage destroys the prestige of a site in the public's eyes, thus reducing would-be traffic
3) Are spammers so desperate to still believe that the few remaining visitors are going to hop on their spammy links and buy whatever they have to offer?
4) With project honeypot and similar databases out there to identify spammers (and their IP addresses) there has got to be some fallout to the spammers themselves. It takes some time/effort to keep switching IP addresses and such.
5) This is pure speculation, but google is smart enough to identify spammy links; why wouldn't it also be smart enough to follow the thread to the source and devalue the source as well, thus causing a horrible side effect of comment spam?