It doesn't use them to insta-spam an email, which I think is where blacklists go wrong. It gives spam points to the message if the sender is on the lists, which turns them from "blacklists" into "suspicious lists".
If an otherwise normal email happens to come from a blacklisted computer, it'll still have a chance to get through, which is the correct thing to do, in my opinion.
I still believe that everyone has the right to run their own SMTP server, and I dislike that so many places blacklist someone just because they are on a cable modem.