Rick Jelliffe at the O'Reilly XML Blog - XML Schemas: guaranteed non-interoperability as a design methodology?:

"Why not? Because, as far as I can make out, the idea that we will all be better off if we pretend that XML Schemas is a unified and whole specification, one size that can fit all, then somehow it will magically happen. But fantasy is a really poor substitute for reality. Time and time again I have seen clients happy about XML Schemas and its promises, only to have their hopes dashes as they realize that as soon as they need to start deploying they have to use subsets and there is no support from “standards” to help interoperability."