I do not span anything game wise over 2 monitors.
I do however have all stats/comms/relevant info on the secondary display.
So, without interrupting my game I can read a guide in a web browser. See friends who are online in Pidgin/Steam/Skype (And any IMs they do send) Can review all critical system data (Fan Speeds, CPU/GPU Temps&Load, FPS, Voltages).
Very very useful even without there actually being a game on both monitors.