The most valuable thing XMPP gains from using XML is namespaces. The way namespaces have been defined to work on top of XML, is what allows the protocol to be d

The most valuable thing XMPP gains from using XML is namespaces. The way namespaces have been defined to work on top of XML, is what allows the protocol to be distributely extensible. I.e. anyone can define new protocol without having to clear this with a centralized body, not even the XSF. If you'd want this property in a JSON-based wire protocol, you'd invariably going to end up with XML-in-curly-brackets. This is why developing with/for XMPP is slightly harder than a protocol that has a limited feature scope. However, as Dave mentions elsewhere, one shouldn't have to deal with the wire protocol directly to use XMPP in your application. A proper library should abstract from that. Arguably in some platforms, that's still a weak spot.