Why is it forbidden to open multiple namespaces at a stretch?

It's possible to do using namespace foo::bar; (i.e., using the inner namespace without using the outer namespace first / at all), why does the standard forbid to do the following?

namespace foo::bar {
  // open nested namespace bar in foo and extend it...
}

I'm not looking for a workaround, just a possible rational on why this isn't allowed.