What's in a namespaceURI?

On more than one occasion we had heated debates about whether a namespaceURI may/must be resolvable. In the session on linked Data TimBL made it clear that he thought that all namespaceURIs must be resolvable. This conflicted with my memory of the namespaces in XML spec which I remembered as saying the the namespace was simply a name (indeed there can be problems when software attempts to resolve such URIs). So I turned to Namespaces in XML 1.0 (Second Edition) which is more recent (and which I hadn’t read) and I’m not sure I’m clearer. I can find:

“Because of the risk of confusion between URIs that would be equivalent if dereferenced, the use of %-escaped characters in namespace names is strongly discouraged. “

and

” It is not a goal that it be directly usable for retrieval of a schema (if any exists). Uniform Resource Names [RFC2141] is an example of a syntax that is designed with these goals in mind. However, it should be noted that ordinary URLs can be managed in such a way as to achieve these same goals.”

So this sounds like “may” rather than “must” be dereferenceable.
Now namespaceURIs also exist in RDF documents (whether or not in XML format), and Tim was very clear that all URIs must be dereferenceable. I don’t know how whether this is formalised.
Looking for RDF I find Resource Description Framework (RDF) / W3C Semantic Web Activity which contains:

“The RDF Specifications build on URI and XML technologies”

and the first links contains:

“Uniform Resource Identifiers (URIs, aka URLs) are short strings that identify resources in the web: documents, images, downloadable files, services, electronic mailboxes, and other resources. They make resources available under a variety of naming schemes and access methods such as HTTP, FTP, and Internet mail addressable in the same simple way. They reduce the tedium of “log in to this server, then issue this magic command …” down to a single click.

All documents date from 2006.
So I think there is “XML namespaceURI” and RDF namespaceURI” which if not identified separately are confusing. Or maybe the time has come to make all namespaceURI dereferenceable even if their owners assert they are only names. In which case what is the value of the resource? The simplest should be the “Hello World!” of the URI:

“Hello Tim!”

I shall try to make namespaceURIs resolvable although this is difficult when not connected to the Internet.