Consider a rather big provider that allocates IP addresses dynamically to its customers. The adresses are taken from a finite set, and if the customer doesn't connect for some time, her address is affected to someone else.
Likewise, when she reconnects, her new address will be taken from the stock of available IPs. Nothing new, but think of it twice: it means that if the previous owner of my IP address was a heavy spammer, I get immediately blacklisted by hundreds of mail and web services in the world.
To me, this shows that relying on RBLs may not be such a good idea as it seems. The method you describe is good for oldtimers who want to protect themselves from the mass (those who don't have a fix IP address) but it doesn't serve well users who may receive emails from the real world.