Dan Brickley comes out with a stonking good post to atom-syntax succintly listing all the reasons why RSS 1.0 adopted the RDF model:

• (fairly) predictable XML notation; RSS 1.0 defined a profile of the RDF/XML syntax, so that namespace-extended feeds all shared a basic structure. (rather than allowing all RDF's syntactic variations).
• Supported free combination of independently developed descriptive vocabulary (manifested as RDF/XML-based namespaces). RSS 1.0 feeds can carry extra markup describing things in the world beyond syndication, such as people, places, movies, bank accounts. Element names in the markup correspond to classes (categories) and properties (fields, relations etc) defined by any RDF vocabulary that proves useful.
• The external vocabularies a feed draws upon do not need to be defined with RSS 1.0 (or Atom or newsfeed syndication) in mind. Or be tightly coordinated amongst themselves. There is a tightly-defined model and simple-minded (additive) model for explaining how these independent namespaces interact when deployed together.

Dare Obasanjo then misses the point entirely, failing to see that just allowing arbitrary namespaced elements in RSS is not the same as adopting a well thought out data model.