We may need more info. Some games like supreme commander support dual screen, but I beleive you mean having a naturally single screen game spread over a dual screen res?
I have 2 screens and would never dream of gaming over two. Game on one screen, or three.
Let's be honest, for FPS it is useless as the crosshair would be over the bezel. On other games it would just be annoying. I imagine there are very few games that work over 2 screens - Perhaps strategy games where one screen is the game and one screen is a map or something.
To be fair, loads of games would, you could have the HUD on one and the main screen on the other. Racing games could have track map, positions, times etc on the other screen. RPGs could have skills, inventory etc. Even FPS could have the scoreboard on the 2nd screen.
I think this is the main problem, in that to get full value from it the actual games themselves would need to be specifically coded to take advantage of additional screens, i.e. ability to put different elements onto the second screen (all the examples you cite are good, RTS would see big benefits too).
That won't happen in many games because it would require a big investment from developers for a feature that 90%+ of players won't use. I've used dual monitors for years but not for gaming, if I'm running a fullscreen game then I tend to have e.g. Mumble and MSI Afterburner on the secondary monitor to negate the need for overlays.