Matthew Gertner has written a well thought out piece describing why schemas are going to be important for widespread adoption of XML. The basic thread is that schemas simplify things for the end user by enabling tools to become smarter. He dismisses RDF along the way, but in my opinion RDF is never something you'd want to write by hand, it's better generated from a database. But, the whole point of the article rings very true, XML will never succeed unless it is kept simple, and that applies to the tools needed to create it as well.
The Big Idea is that HTML beat SGML to a pulp in the market because it is so damned simple. The XML 1.0 Recommendation does an outstanding job of preserving the elegance of SGML while eliminating countless optional features whose complexity outweighed their benefit. Now it is up to the XML Schema WG to complete the picture; if they can follow the lead of the original XML authors, its future may be very bright indeed. But let's not forget that we aren't quite there yet.