In some people's mind (maybe a strange place), a namespace is a name and so using URLs is bad!
Other people say that URI needs to be resolveable and URN's are somewhat disfunctional in terms of resolving them to a resource.
Well respected people have been changing my thinking on using URNs because:
You can't get a NID.
You always need a catalog to resolve URNs as there is no default resolving mechanism.
It is really super nice, actually fabulous, to type a namespace name into your browser and get information about the schema. ...maybe even the schema itself.
The W3C doesn't use them and so why should you?