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> Thanks for the clarifications, and I agree these technologies
> need to coexist. It would seem one could start from the
> XML DTD/Schema types, get the local systems running, then
> come in later and add RDF/OWL type descriptions.
> What are the benefits of dual systems of classification?
Take at look at http://www.daml.org and a few sample ontologies for starters. There are pointers to a number of examples. I'd say that for the examples given, its alot easier to express the intended relationships and classes than in, e.g. XML Schema which isn't designed for this in specific. To be more specific, there are already industrial strength ontologies e.g. SNOMED which are created under the 'description logic' (DL) formalism that DAML+OIL has captured. There are also industrial strength, as well as open-source/free DL classifiers e.g. Classic, FaCT etc. that already work with such ontologies. Bolting on DAML syntax input, which allows such ontologies to be shared over the Web, wasn't all that hard.
So you might ask the same question from the other perspective e.g. "Well I already have a good relational database system, that already has a data definition language, so why do I need DTDs or an XML Schema?"
In the case of OWL, suppose I want to express something like: "If you are the father of me, then I am the child of you". Piece of cake for OWL, but how would you say that in XML Schema, or a DTD?
On the semantic web, we'd like our system to figure out these braindead sorts of things, rather than miss the information that is staring us in the face, just because it was coded in the wrong column etc.