One comment I would make, is that any well designed software defines it's interfaces. XML is effectively an interface, so it's format really should be documented. To this end, XML Schema and DTDs are very effective and I would say very worthwhile.

Validating source documents against your XSD/DTD at runtime could be considered overkill (and could just throw any performance you were hoping to have out of the window)

My opinions are purely that, and are based on working on a very large multi component XML system that I've been involved with the design of for the last year or so.

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RatArsed