Having said that, I still signed up for the beta. Because I'm a primadonna.
I would love to see a system that does automatic spam classification (like gmail), but if you get auto-classified as spam, then you get the challenge-response, as a last ditch effort to get through the spam filter.
This is why it's generally bad practice to reply to an email telling the sender it is spam.
It depends on what you prioritize. I prioritize avoiding false-positives, so my spam filter of choice reflects that.
My current spam system (spamstopshere.com) does return bounce/reject messages depending on what filter the email triggers.
As for the original idea, backscatter is to be avoided at all costs. Nobody likes bounce messages and so forth for mail they never sent. Much better to reject mail at SMTP time, than generate backscatter to the alleged envelope sender after the fact.
I get a lot of spam, so manually reading a spam folder is not a feasible solution.