I was thinking about using this format for the REST/XML version of the PeopleAggregator API, but I had decided against it because it would be inconsistent with the XML-RPC and REST/JSON versions. But now that it seems that everyone else is using it, I guess I should reconsider :-)
Update: Michael Fagan encourages basing APIs on RSS/Atom + Atom-Pub + OpenSearch, i.e. like GData. That approach is appealing, although I expect it would be significantly more difficult to code against than an XML-RPC or JSON API.
Update 2: Looks like I got Marc worried - I've clarified this post over in his comments.