Why URI?
The only requirement for a namespace definition is to be unique within all the applications, it possibly can be used in. There is no need to specify a URI scheme, or some www... subdomain.
Namespace Versioning
Major changes in the schema of the namespace would result in a new version of this namespace. So the namespace URI needs to specify a version.
Namespace URIs
RFC 3986 - Uniform Resource Identifier (URI): Generic SyntaxEach URI begins with a scheme name that refers to a specification for assigning identifiers within that scheme.
Ballot on URIs and Namespaces
http://www.w3.org/2000/09/xppa

Ballot on relative URI References in namespace declarations (3-17 July 2000).

"This specification does not define an information set [or whatever] for XML documents which use relative URIs in namespace declarations."
W3C XML Plenary Ballot on URIs and Namespaces, 29.8.2010

Does not show any advantages of specifying relative namespace URIs.

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