People hate namespaces wherever they are used, but they are one of the keys to "programming in the large". The first year after Java came out the official docs for namespaces weren't complete and I had to go to a web page at NASA to understand exactly how they work, particularly in the strange case where you don't include a package statement -- people got used it though. (I'll make the case that Java was a software reuse revolution not because it was OO or had a particular implementation of OO but because of getting a bunch of "little" features right such as namespaces and memory management.)
If we had namespaces for classes and id's in CSS we'd be able to transclude content from one HTML to another HTML page easily but we don't so we can't. (Shadow DOM helps a bit though) The "hygenic macro problem" is one of the many barriers people face doing metaprogramming. In the RDF world it's refreshing to be able to import facts from multiple systems into multiple namespaces so you can put data you got from different sources into the same database and "it just works".
A certain amount of namespacing is useful, but the XML model where you can't even write a hello world xpath query until you've set up all your namespaces and used them in your query is something that clearly does more harm than good. Java is I think widely recognised as having gone too far on the namespacing front; most post-Java languages have a much less strictly hierarchical namespacing model and I don't think any other package registry has adopted the fully nested style that Maven does. And even Java's heavyweight namespacing is much easier to work with than XML's thanks to things like auto-import in IDEs.
I don't think namespaces solve the macro hygiene problem, because if you're going to have macros then they need to be able to interact with and manipulate your namespaces, and sooner or later you need your macro to e.g. generate a fresh identifier for use within an existing namespace and then you're right back where you started.
Being able to disambiguate when you have one document including another would be useful. But the cost/benefit on XML-style big namespacing up-front just doesn't stack up.
If we had namespaces for classes and id's in CSS we'd be able to transclude content from one HTML to another HTML page easily but we don't so we can't. (Shadow DOM helps a bit though) The "hygenic macro problem" is one of the many barriers people face doing metaprogramming. In the RDF world it's refreshing to be able to import facts from multiple systems into multiple namespaces so you can put data you got from different sources into the same database and "it just works".