As an aside, this article seems quite devoid of actual content, bordering on spam IMHO. The title mentions IoT but the term appears only once in the entire article at the end, in a vague "XMPP is for messaging and IoT also needs messaging, so.... XMPP and IoT!" way. This is the sort of page that clogs search engine results with useless non-content.
If one complains about extensible protocols, one should be willing to come up with some other approach that works.
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.
Why you might want it? Well, maybe for the same reason any home router has a built in web interface (despite all the overheads of having a kinda webserver on such device) - for user's convenience and interoperability.
And yes, stating that JSON is somehow better than XML for such cases is just... wrong.
If you want to IoT, why not just use NATS or something?