Why bother with this?
http://www.infoworld.com/article/04/08/31/HNspammerstudy_1.html
Did you read the end of the article? SPF prevents forgery, not spam. It's still valuable even if spammers use it.
Why bother with this?
http://www.infoworld.com/article/04/08/31/HNspammerstudy_1.html