It takes less work and risk to promote a feed from an authority site like a social bookmarking site than it does www.myspammy.info/spammy.xml.

The links in both feeds go to your pages, they are just organized different. Subtle but important difference. Instead of the search engine seeing Kurt's feed of his own links promoted by Kurt, they see Someone Different promoting a feed of links from Kurt (and maybe others).

So far there hasn't been a devaluation or penalizing of links in social bookmarking sites like there was in directories or guest books. For some reason, the search engines are relying on the bookmarking sites to keep themselves spam-free.

Very similar premise to why Squidoo pages sometimes outranked the actual site in the SERPS. And we have seen that change not too long ago but it is still effective.