Why not? : )
In all seriousness though, having tried both dual and triple card configs for my triple screen setup, I vastly prefer using 3 cards with enough VRAM to handle higher quality textures(Obligatory Skyrim Reference). Single screen it is entirely wasted, but in BF3, on high/ultra settings, I get between 70 and 105 FPS on 5760x1080, with a minimum of 65fps or so, which, even if most people say you can't really see, I can feel a difference. And with how cheap you can get them versus 4GB 680's plus my budget at the time, it all kinda worked out.