There has been an interesting discussion on the OASIS member-discuss list (members only, I'm afraid) regarding the use of a namespace document.
If you are following the activities of the W3C TAG, then you probably have seen their draft finding on Associating Resources with Namespaces. If you haven't read that yet, you should.
Robin Cover has done an admirable job (as usual!) of collecting all things RDDL in a Cover Pages entry here. Hopefully, this will help raise awareness within the community of those developing and publishing XML namespaces (whether publicly or privately).
You may have noticed that the WSDL1.1 Binding Extension for SOAP1.2 spec published last week made use of RDDL for its namespace document.
My personal opinion is that use of a namespace document is a "Good Thing", and I am glad to see that its use is growing. It is my hope that the OASIS TAB (full disclosure, I am a former recovering member of the TAB) will make a recommendation that its use become a recommended practice for OASIS TCs.
RDDL me this, Batman