Why is namespace composition so rarely used?

Question

In his book The C++ Programming Language (third edition) Stroustrup teaches to define individual components in their own namespace and import them in a general namespac

Answer 1

Mostly I wonder what the benefits would be (as Raymond Chen says, every feature starts with -100 points). However, I have a counterpoint to offer: Luabind, which does use something that looks like this. See luabind/object.hpp, which essentially says:

namespace luabind {
  namespace adl {
    class object {
    };
  }
  using adl::object;
}

From the name alone we can infer the motivation: to support argument-dependent lookup. Given what the user knows as a luabind::object, which is actually a luabind::adl::object, related functions will be discovered automatically by the compiler from the luabind::adl namespace. Yet those functions, which the user may not need to know about in a very explicit way, do not "pollute" the main luabind namespace. So that's nice, I guess.

But you know what's not nice? Forward declaring one of these classes. This fails:

namespace luabind { class object; }

You need to do this instead:

namespace luabind { namespace adl { class object; } }

Thus the abstraction quickly leaks, as the user does need to know about this magic adl namespace after all.