I see your point. Some of my friends had badly positioned center channels (due to the aesthetics of placing the center channel). In those cases.. .the surround system sounded much better when you turn it off. that way the 2 main speakers take care of the vocals. Also in systems where the center is too small, and the main speakers are bigger. It sounds better just using the main speakers instead of the center.
My guess the center is probably for imaging for viewers that aren't completely centered. If you're not in the sweet spot between the 2 fronts, your imaging isn't going to as good.