Most games take priority and as many of you know, when playing most games and you alt-tab it unloads the game and brings you to the desktop. With Outerra this does not seem to be the case, and this would cause a performance hit. To illustrate what I'm talking about, here is a pic:
See how the game continues to run fine in the background while other things are up.
Some may say that that seems like a feature you'd want, but when a application does not have full screen priority it's essentially running in windowed mode, which never runs as well as fullscreen.
Others may point out that people with 3 monitors are such a small portion of customers that it is not especially important to keep them a priority, and most games take this attitude. With Outerra, it is a simulation game that is targeted at simulation audience, and if I know anything about people who play simulation games, it is that a good portion of them have 3 monitors.
Right now I am using the Nvidia surround method, as I don't have an ATI eyefinity card.