Yeah, I remember an old DOS game that used two mices. Settlers 2 strategy game had a multiplayer split screen, requiring 2 mices (I remember to try it with my brother
Anyway, I think SFML interface for single mice, multiple mices and touch-screen should be the same for all. And it's still possible to simulate touch-screen on OS that supports only one mice.
So letting a middle class being extended would allow SFML users to add their own touch-screen simulators or way to reinterpret devices input, being it able to incorporate even other input devices that aren't directly supported in SFML (well maybe it isn't that easy to generalise that much, but eventual devices similar to mice/touchscreens can easily be incorporated).