There's been quite a bit of discussion on the mailing list around the question of using XML schema with Atom. There are incongruencies, notably that to get good WXS support the order of elements with Atom can't be arbitrary.
Dare Obasanjo has provided some initial bullet points of what a vocabulary gets from having an XML Schema :
* Usually provides a terse and concise description of
the vocabulary [relative to the prose of the spec]
* Enables software to validate that XML documents
being received from clients or servers actually
conform to the vocabulary. This prevents issues like
each application hacking up its own validator or
"liberal RSS parser".
* Allows vocabulary to co-exist with technologies and
tools that already support features specific to a
schema language such as relational to XML mapping,
object to XML mapping, directed editting, etc.