XML Namespaces aren't mandatory, and tools shouldn't assume that they are.
In his recent blog posting on XML Namespaces, James Clark questions the universal goodness of namespaces. Of course, there is plenty of goodness there, but he's right to question it. He says the following:
For XML, what is done is done. As far as I can tell, there is zero interest amongst major vendors in cleaning up or simplifying XML. I have only two small suggestions, one for XML language designers and one for XML tool vendors:
For XML language designers, think whether it is really necessary to use XML Namespaces. Don’t just mindlessly stick everything in a namespace because everybody else does. Using namespaces is not without cost. There is no inherent virtue in forcing users to stick xmlns=”…” on the document element.
For XML vendors, make sure your tool has good support for documents that don’t use namespaces. For example, don’t make the namespace URI be the only way to automatically find a schema for a document
It's the second point that interests me. During a recent Tridion project, there was a requirement to accept data from an external source as an XML document. I wanted to use a Tridion component to store this data, as this would give me the benefits of XML Schema validation, and controlled publishing. The document didn't have a namespace, although it had a schema. In order to get this to work with Tridion, I had to go to the provider of the document, and get them to add a namespace. Tridion wouldn't allow me to create a schema whose target namespace was empty. It seemed a shame that even when hand-editing the schema (so presumably asserting that I knew what I was about) the system wouldn't let me make this choice.
At the time, I just got the other party to make the change, and went back to more important things. Maybe there's some internal constraint in the way Tridion works that prevents them from supporting this, or maybe it's such an edge case that no-one was ever bothered by it. If the former, then I can't think what the problem would be; there's no reason to abuse the namespace to locate the schema. Tridion's quite happy enough to allow several schemas targetting the same namespace, so what's so special about the "no" namespace? In Tridion components, XML attributes (quite correctly) are in no namespace, but as long as the correct schema gets used for validation, so what?
I suspect it's more likely that this just comes under the "edge case" heading, in which case, perhaps they can improve it in a future release.