Multi-mon isn't a big advantage in RTSes cause none of them have done it right.. the only game that has come close is Supreme Commander with its dual monitor support - you get 2 viewports, 1 per monitor. Unfortunately the secondary monitor was missing all the HUD elements.
Even with distortion at both extremes, I think it´s and very considerable advantage, much more than in shooters (though less than flight sims).
In DoW2 and CoH, I still need to move frecuently the camera, and I can´t control perfectly units at the very left/right side of lateral mons, but I can watch way more of the terrain and get a better idea of what It´s happening now in the terrain, wich I think It´s a clearly advantage (in DoW2 In can see from corner to corner in some maps). For that, I sit way more ahead than usual for FPS. Peripheral vision is not important for strategy games, extended vision is.
Proper multimon would be better, of course, I agree with that.