spammer spams for an entire weekend until his account is yanked,
I've picked that fragment out of context only as an example of the larger religion that holds "most spam is forged" as an article of faith.
Most of it is. But that's not an incredibly interesting point: I think we can all agree that there is a lot of spam that is forged and there is a lot of spam that isn't forged. There is too much of either type.
It sometimes seems like people are saying "it's no use going after spam that's forged as there's always the non-forged type" but then also "it's no use to go after non-forged spam because spammers simply forge headers to get by filters". May I suggest taking logic 101 again for those of you who think this makes sense?
If you watch much spam without religious blinders, you know that the religion that enough ISPs care about spam is delusional.
If an ISP doesn't yank the mail identity from people that spam and/or lets spam through from known spammers, that would be a good reason to require additional validation for identities used by customers of this ISP.