Although stringstream inherits from iostream, it is not declared in the iostream type along with the famous cout, cerr, cin, and clog types, but not other types that are iostreams (for example, file streams). For these, you do need to explicitly #include the requisite header files.
EDIT: In response to your revised question, I pulled up the C++ spec and interestingly it does not say that #include without actually getting a full class definition for either istream or ostream. Only explicitly including those headers can guarantee that the definitions of those classes, not just the forward-declarations, are visible.
