Tim Bray has two highly diggable posts on writing more XML schemas instead of reusing existing ones. His advice is: Just don't do it. And if you do, do as little damage as possible. I feel his pain daily. The domain business (in which I work) is a world with tons of vendors all of whom have APIs of some sort, all of which API's do more or less the same thing and frightfully many of which API's are different, sometimes ever so slightly, but enough that you need to handle each case on it's own. There's even an established standard - but of course only the most stringent and controlled ov vendors abide by the standard. Most add their own gunk to it. In the best case, this is done in a stringent and documented manner, but occasionally it's just broken and badly so (when it gets really bad the XML is not even XML).
Also, this post is a reminder for myself about the acronym YAGNI.