the reason you don't see a lot of spam on IETF lists is because it's sent to the list administrators, and they filter it by hand.
The chief beneficiaries of automatic spam detection and deletion in the current IETF setup is the list administrators.
I'm one of those list administrators and I can attest that having spam flood the review queues of the mailing lists is a huge problem. It's not terribly unusual for the review queue of some lists to get so large that you can't download and resubmit Mailman's review page without crashing the web browser (and I've tried several different browsers on different platforms).
but despite first-hand experience with the problem I'm still worried about using SpamAssassin - I've seen it block too many legitimate messages.