Producers and consumers that support RSS 1.0's model can uniformly interpret that information. A system migrated to Atom without support of this kind would lose a significant facility.
Prior syndication art - RSS 1.0 fully supports it, e.g. there's a WordPress plugin to add terms from FOAF to give more info re. topics, authors etc:
http://www.wasab.dk/morten/blog/feed/rdf
http://npg.nature.com/npg/servlet/Content?data=xml/02_newsfeed.xml&style=xml/02_newsfeed.xsl
Atom will (hopefully) have a syntax that will be considered simple. There is no suggestion here that it should support a model like RDF, just some form of mechanism or convention that will allow it to do a reasonable proportion of what RSS 1.0 can do.