When I'm building firmware for an ATMega or a Cortex-M0 or Tensilica device, the idea of dedicating the required space in my firmware for an XML parser, burning the CPU overhead for doing the parsing, losing the network overhead for the verbose text encoding... all this is absolutely absurd. Just so I can use XMPP? Why? That's my battery life getting wasted, that's my features getting displaced.

There's no place for XML in iot, and definitely no place for XMPP. Even json is luxury you often can't afford, not if you want to make a reasonable cost target or battery life.