The main difference is that the "horizon" of the sound stage is level with the L/R. This is more important for music, but also for movies. When I used only one, I had the choice between an orchestra that was layed out "/\" or "\/" (not as pronounced, but you get the gist). With one VP150 shooting at my stomach and one slightly above my head, the sound stage becomes flat.
One of the best way to "hear that" is to set your system 4.1 and listen to the front stage of multi-ch and then move to 5.1. With dual centers, the "horizon" remains identical, with a fuller front stage. With the center at the bottom or top, the "line" is skewed.
