“While extensibility of a given XML DTD or schema must be designed into it from the start, RDFS and OWL schemas allow a lot more flexibility and therefore more possibilities to build on the work of others.”
“… it’s much easier to combine multiple RDFS/OWL schemas for a single application than it is to combine multiple XML schemas/DTDs.”
http://www.snee.com/bobdc.blog/2009/04/semantic-web-technology-and-hu.html