Remember that the spammer is handing the "open" relay one piece of mail with zillions of RCPT TO:s - rate limiting the outbound just means that the zillions of recipients sit in *your* queue that much longer.
I will add "further punishes/diminishes the harm of open relays" to the "pro" column. ;)
I have heard from multiple sources that the spammers are well clued enough to utilize multiple relays in parallel - if you rate limit to 1/N of bandwidth, they just use N relays at the same time.
If the rate limit is applied to *:25, then the *MORE* relays the spammer uses, the longer it takes for him to do it. -- Eric A. Hall http://www.ehsco.com/ Internet Core Protocols http://www.oreilly.com/catalog/coreprot/