There are convincing arguments against using namespace std, so why was it introduced into the language at all? Doesn't using namespace defeat the purpose of namespaces? Why would I ever want to write using namespace? Is there any problem I am not aware of that is solved elegantly by using namespace, maybe in the lines of the using std::swap idiom or something like that?
For one thing, this is the way to use operator overloads in a namespace (e.g using namespace std::rel_ops; or using namespace boost::assign;)
Brevity is also a strong argument. Would you really enjoy typing and reading std::placeholders::_1 instead of _1? Also, when you write code in functional style, you'll be using a myriad of objects in std and boost namespace.
Another important usage (although normally one doesn't import whole namespaces) is to enable argument-dependent look-up:
template <class T>
void smart_swap(T& a, T& b)
{
using std::swap;
swap(a, b);
}
If swap is overloaded for some type of T in the same namespace as T, this will use that overload. If you explicitly called std::swap instead, that overload would not be considered. For other types this falls back to std::swap.
BTW, a using declaration/directive does not defeat the purpose of namespaces, since you can always fully qualify the name in case of ambiguity.