Here are a couple that immediately come to mind:
1. Aesthetics - people are used to seeing a flat center channel in an HT application. The look of two vertical centers (even if they were no taller than a regular center) may strike some as weird looking.
2. Shielding concerns. While the M2 and M3 seem to be immune to this problem, it could cause a problem with lesser speakers.
3. Dead zones or null points - the center channel will aim right at the prime listening position, in a perfectly parrallel configuration. Having two speakers toes in or out may create hot zones or null spots where the signals cross. While this problem can be fixed by decreasing or increasing the amount of toe-in, the casual use may dislike having to monitor this center channel with regard to toe-in.
Keep in mind, though, that while this idea isn't discussed - another well marketed company (read the "BLOWS" cube system) has been selling a center channel cube where the top and bottom unit can be flayed left and right, respectively so as to capture a larger sweet spot in the center listening position.
