People working on RSS 2.0 ran into the reality of XML Namespace handling.  Bottomline is that not every XML software knows how to handle namespace properly.  A similar problem was encountered with 3D-Secure message format which is XML-based.  3D-Secure had to use XML-Signature so namespace had to be supported, yet a key partner company's product ignored namespaces.  The solution was to require that only default namespace declaration is allowed and forbid the use of namespace prefix.  The decision to have no namespace for 3D-Secure elements was arbitrary.

My recommendation for RSS 2.0 is to support namespaces but not require them to be declared unless they are needed and require only default namespace declaration to be used. This would allow legacy RSS software that can't handle XML namespace declaration attribute to continue working.  While this is a hack, but it is a practical hack since the primary value of RSS format is that it is being used widely and anything that breaks that voids the value of RSS.