Tons of highly popular languages don't have XML parsers built to the spec. Very few of them can handle schemas at all, or recursive definitions in schemas, or external document types, or external entities or schemas (and the few that can are usually disabled intentionally, because they can make parsing a tiny XML file take minutes if the server of the schema doesn't serve it up fast enough). That the XML spec defines such things is irrelevant if they're wholly unreliable because enough parsers refuse to handle them. And there are more than enough horror stories out there to argue that a significant amount of systems don't handle to-spec XML.

There are plenty of RFCs out there that define things that no longer practically exist - should they be used, because the spec defines something that you need? Especially if it's meant to be used as an interchange format?