Well that's the point, isn't it? XML unifies various independent ecosystems. At the very least, if defines such essential things like data and time representati

Well that's the point, isn't it? XML unifies various independent ecosystems. At the very least, if defines such essential things like data and time representation formats, whereas JSON doesn't even attempt to do that. But then XML goes further and provides you with a contract. Now anyone can write their independent implementation and test it (validate it) to that contract. This is what Web was about in the first place. You can use any technology imaginable, any language imaginable, but in the end you are delivering to common format, HTML+JS. Now that this is insufficient, the natural evolution would be XML and REST.  JSON, however, is a liability. I believe it is, in fact, the next IE6!