As much as I can agree that XML doesn't prevent user adoption, it may very well prevent developer adoption; if developers are not willing to bother with XML or any part of the protocol, they may very well give up and develop for some other platform.
Yes, that may sound futile, but in the end the developers are the people who build everything. It is my strong belief that HTTP, IRC, SMTP and bittorrent (among others) have thrived because of their utter simplicity, to the point we're embarassed today because we've ended up with so much under-specified crap on top of them. Still, they deliver. As always "worse is better".
Wait, wait. We're talking underlying data format here.
Are you saying that serializing something to JSON (or whatever you fancy here) vs. to XML .. saves battery?
I mean, XMPP is certainly not the battery friendliest tech right now (elsewhere people discuss push extensions for example), but .. that's not related to the use of XML.
Having to support a panoply of marshaling standards would suck battery more, not less. I doubt XML vs. JSON vs. YAML per se makes any substantial difference in CPU usage. You might get a little mileage in going to a compact binary format and reducing data transmission, but is that worth it?
Are you saying mom and dad aren't using XMPP because the message is sent using XML based stanzas? Facebook is ditching XMPP because of the X?
It doesn't matter?