The reason that Yahoo and AOL are looking to charge for e-mails isn't to foil spammers. It's a fairly common misconception that spam comes from a guy with a server cranking out e-mail all day. There are some of them but they generally get their mail blocked pretty quickly. Spamhaus, Spews, Spamcop and OpenRBL do a pretty decent job of tracking these people and making free blacklists available.
Generally spam comes from your neighbor's unsecured machine. Typically you'll find if you signup for cable modem service, they hand you a modem and a usb cable. When you hook everything up you're left with non-firewalled computer with a public IP address. It's then just a matter of scanning the IP ranges of cable modem and DSL providers looking for exploitable machines. They then redirect spam at those machines and proxy it to the world knowing that the blacklist services can't possibly tell everyone to block all e-mail from a provider like comcast.
AOL and Yahoo didn't actually come up with the idea of charging for e-mail. Large legitimate mailing companies have been after them for a long time to do so. They are willing to pay a fraction of a penny per e-mail to ensure that they aren't effected by a particular provider's spam filter. When say The Gap pays it's mailer to send out advertisements to everyone registered on their website, they are willing to pay a small percentage more to ensure that the message gets delivered.
The real question that everyone should be asking is how long will it be before AOL starts selling it's customer's e-mail addresses with full demographic info. Or selling the ability to e-mail a particular demographic, say all users who are male 18-34.