|Just another Perl shrine
I doubt the more than a million AOL customers can be considered to be on a single LAN. Sure, they have "lots and lots" of proxies, but up to a few years ago, they hid behind a dozen proxies. Perhaps they now have a few hundred of them. That still means they have a high user/IP ratio. And AOL isn't the only ISP with a lot more customers than proxies and/or NAT boxes.
But you shouldn't only think big. Think small as well. Think families. Lots of families either share a computer, or are hidden behind the same proxy or NAT box. Blocking on IP means that little brother can vote, but when he shows the site to his sister, the sister can't.
It's user hostile, and the tell mark sign of a bad programmer. A good programmer would never start from a broken design.