RDF and dc:subject
I can state that both URLs have the subject "The battle of Hastings". But text strings may have different meanings, even if they are equal.
I can state that my article has dc:subject http://en.wikipedia.org/wiki/Battle_of_hastings. If that is the right way to do it, then the Wikipedia article must also have its own URL define its dc:subject. But that means that the article is about itself. Some pages are about themselves, e.g. Wikipedia on itself and index.rdf. But the article on the battle of Hastings is clearly about the battle, and not about itself.
I end up wanting a new Dublin Core property, say dc:hasSameSubjectAs. But maybe I'm just not well enough versed in RDF.
Illustration from W3c's validator:
Note that 3 occurrences of the URL http://en.wikipedia.org/wiki/Battle_of_hastings is drawn as one object, while the 2 occurrences of the text "The battle of Hastings" are two distinct objects. That is the essence of the difference between URLs and tags: Two identical URLs refer to the same object, while identical tags may be so by accident.