A XML namespace should be a URI, but it can use any URI scheme, including those that are not URLs.

Then why do all widely used XML namespaces use the http scheme (e.g. http://www.w3.org/XML/1998/namespace), considering that trying to use the URI as an URL by retrieving that document using the HTTP protocol is not guaranteed to do anything useful (and often doesn't)?

I understand the usefulness of DNS domains in namespace names to help guarantee uniqueness. But that doesn't require the http scheme, there could be a separate scheme for that (something like namespace:w3.org/XML/1998/namespace). This would avoid the confusion between URIs and URLs, while maintaining domain-based uniqueness.