It's easy to forget that there is a large set of different developers. And some don't know what you may consider common knowledge.
IMO, something being JSON over XML would be a significant plus in my mind.
Something about XML just has never really sat well with me - whereas JSON just works when it comes to actually modelling objects in a way that makes sense and can be picked up quickly. I can't say I've yet to find a place where I absolutely needed a feature of XML that I couldn't find in JSON, but would love to know more situations where I can be proven wrong.
I think ultimately if the goal is just to JSONify the world, then that's no good - but if we're looking to make protocols more accessible and hackable, JSON may be the way to go.
With regards to schema/ validation, wouldn't it be a poor choice to put this logic in the transmission layer?
But besides the point, XMPP already uses XML. So there is no need to reinvent the wheel here.