Outbound links and negative SEO

Forums and blogs generally use nofollow for outgoing links so as to ensure that links added by people other than the owner do not receive any PR from their site.

One billion nofollow links to a directory is worth nothing as far as PR is concerned

Really? It seems like they get a lot of inbound links on forums and blogs. I know those links aren’t worth a whole lot, but it just seems like their Page Rank is a little disproportionate to me.