JH, probably the main advantage of having a separate speaker to handle the center channel sound is that the sound(notably dialog in movies)will be anchored to the screen in the center even for listeners who are sitting away from the center. Using the mains to reproduce the center by using the "phantom" effect works well for someone sitting in the center, but as the listener moves away from the center the center image moves to the side with him(but not farther out than the main on that side). Another factor is that with a separate center speaker its level can be adjusted independently of the mains, for example raising it a little to try to make dialog more understandable.
You might try using M2s mains alone first to see if the phantom center satisfies you. Then if not, you could later add a center speaker(possibly a vertical M2)to improve the results.
