Not sure what all the fuss is about. It's actually neat to have a common way of changing JSON to XML, think of a web service that offers an API using our beloved JSON. But wait, we also want to provide an XML API! Let's design our own, ad-hoc, crappy mapping or... use that thing that everyone else is using.

XML is out there, has lots of tools and libraries and is actually useful for some things. Not to mention the array of technologies around it, like XPath, XQuery etc, that JSON is lacking (probably because it doesn't need it, because these formats do address slightly different requirements. Gasp!).