I agree that XML is only slightly more verbose than JSON but they should still make a JSON version of the spec or it will die. It should be pretty straight forward to mechanically change the XML into JSON.
:-)
On the other hand, both stanza.io and XMPP-FTW both use the same fixed-format (ie, inextensible) JSON format for XMPP just fine. They can do this because they drop unknown namespaces and don't intend to provide round-trip conversions, etc. So if you're after a JSON "interface" to XMPP, there is one, and it's pretty good.
Worth a look.
EDIT: Also the X in XMPP stands for eXtensible not XML