Why not use mains as centers

I think the only reason companies make special center dchannel speakers is so that they will not take up a lot of height, and fit on top of or underneath the monitor OK. If your speaker fits under or on top of the monitor, I would say it should be better than a center channel. It seems like all the specialized cabinet design that manufacturers tout as a reason to buy their speaker goes out the window with center channel design.

Do people really think the extra effort put into time aligning the drivers, making non-parallel cabinet faces, controlling horizontal dispersion, etc. becomes unimportant in the most important sepaker in the system? You can either think it is unimportant in all of the speakers, or important in all of the speakers, but it doesn't make sense to think it is only important in the side channels, but not in the center. In both stereo and multi-channel, you are trying to get image continuity between an array of speakers - same problems whether you have 2, 3, or 5 speakers.