Hand-generated XML can be just as nice as using hand-generated JSON, and on the other hand automatic domain-model-to-JSON mapping can be as ugly as any XML monstrosity.
Creating idiomatic XML, on the other hand, is a little more tricky. Should something be a tag or an attribute? What should it be named?
Loading a huge JSON file is almost certainly slower than using a SAX parser for a huge XML file. Maybe there are SAX like approaches for JSON, too.
To businesses that need to support multiple formats (enterprise requirements?), XML + XSLT sounds like a fair approach - it allows you to simultaneously create idiomatic XML & JSON
By separating out the rendering-to-an-output format from the basic logic of the application, you get similar benefits without creating a dependency on XML handling libraries and requiring another implementation language (XSLT) for the rendering component.