The strength of xml is xsl. The ability to declaratively transform the content into another form. I'm not aware of any way to do this with json other than by hand-rolling code.
I'm not entirely certain that is a plus... Is writing a compiler/translator in xsl really more pleasant/easier to maintain than writing it in a "proper" language?
I would say that the strength of XML is XPath - forget the rest of XSL (XSLT and XSL-FO) - and I would still avoid it unless there was a real requirement for "document like" content.
That said, I really enjoyed: http://www.amazon.com/Program-Generators-Java-Craig-Cleavela...
edit: for example: you could transform an ant build.xml file -- but should you?