Excellent post by Justin on why SAV isn't that useful in fighting spam:

Some view this as a useful anti-spam technique. In my opinion, it’s not.

Spam/anti-spam is an adversarial “game”. Whenever you’re considering anti-spam techniques, it’s important to bear in mind game theory, and the possible countermeasures that spammers will respond with. Before SAV became prevalent, spam was often sent using entirely fake sender data; hence the initial attractiveness of SAV. Once SAV became worth evading, the spammers needed to find “real” sender addresses to evade it. And where’s the obvious place to find real addresses? On the list of target addresses they’re spamming!