Standardizing an XML vocabulary is fraught with problems according to "Here's What's Wrong With XML-Defined Standards". There is no control of scope, it has too many participants, it takes to long, and they're just too big.
"It is impossible for one organization to orchestrate or supervise standards development in all vertical domains, and we don't expect any organization to announce such plans."
"Without some revolutionary change to the way in which XML-defined standards are developed, the maze of standards will continue to proliferate, and there will be no way to discover redundancies or identify conflicts and reconcile them. At the least, the proliferation of standards will result in millions of dollars of lost effort. At worst, it will corrupt data and compromise business-critical transactions and operations because different parts of the same company will process conflicting XML messages without knowing it. From 2001 through 2004, enterprises worldwide will spend more than $3 billion on XML modeling activities with no return on investment on $2 billion of it (0.8 probability)."
This is a pretty clear outline of why XML is not just the answer. It doesn't matter how schemas are created and interoperability achieved. They have to work together in order for business to operate.
I know I'm not alone in the dislike of Gartner's use of "x.x probability". I heard the some English politician (or someone apparently worth listening to) who said that the chance of going to war with Iraq was "less than 50%". Now what does that mean? They're not making a statement of fact, if it happens or not they're still right.