It is cool, but I don't see the use case outside of browsers. SFML is not meant for implementing browsers (it has no HTML rendering or Javascript interpreters) or browser-based games (it has no HTML5/JS binding), and for regular C++ apps I'm pretty sure a TCP socket is already full-duplex.
If you actually need this for something, there are already a few C++ libraries like libwebsocket which can do this (though I don't know anything about them).