The fact that this representation had to rely on extensions so heavily makes me wonder of the benefits of using the Atom Syndication Format for this representation. Of course, it is entirely possible to argue that, any general purpose feed reader or Atom client can interpret the Atom-specific elements in this entry, and "value-added" clients can interpret the extensions to provide more features. It is true. But this argument is not strong enough.
I couldn't agree more. I can see more value in putting the extensions (as Microformats) into the embedded XHTML for each entry; but otherwise, using a specifically tailored XML format yields about the same benefit in terms of generic tool support as an Atom feed that's full of extensions.