what's so special about center speaker ?

Ya, jd said it right. Horizontal MTM configurations run into horrible/massive cancellations/nulls as you move off-axis to the left or right, since now one of the mid/woofer drivers is closer to the listener than the other. Ironically the whole point of a center channel speaker is to anchor the dialogue for people sitting off-axis. So a classic vertical design is superior, its just not attractive or fitting for many systems. But the marketing guys keep selling'em. You'll see a few designs were the designer was aware of this and tried a little. Dickason's cookbook has a design where he gets the two mid/bass drivers almost side-by-side with the tweeter tucked into the space above where the two drivers begin curve away from each other. But even then its not ideal. So to answer your first question, firstly, they aren't optimized to work as center channel speakers by virtue of their shape, and secondly, and which answers both questions, the more accurate speaker is the more accurate speaker and there should be no real difference between the center and the left and right (in regards to waterfall, impulse response, etc.) The only room for debate is in off-axis frequecy response capabilty which also addresses the phase problems any multidriver system deals with.

Even the MTM design period isn't all that great. Biro Technology has a good article and it appears the designer has found that even a vertical MTM suffers 9-12db dips in frequency response, with what I believe is more ambient energy, assuming your ear level with the tweet. I don't know why there are so many mtm. Granted there's no lobing errors or polar axis tilt, better bass extension and spl, plus higer power handling, those problems can be dealt with in other designs. Needless to say I'm not a big fan any symetric array, whether or not its horizontal or vertical.

To quote Vance Dickason on page 189 of The Loudspeaker design cookbook, 6th ed., "The center front channel speaker is commonly positioned horizontally , with the drivers in opposite acoustic polarity to the left/right front speakers. This is done strictly for aesthetic reasons." Note that last sentence. And on the top of page 189 he has graphs showing the effects of the various driver arrangements and their effects on off-axis frequency response: figure 10.11 correllates to figure 10.15 on the facing page of a horizontal MTM. From the first paragraph in the second column on page 189 "The consequence for the horizontally polarized speaker at 30 degree off-axis is a dip in the repsonse at the crossover frequency of nearly 15db." The other three driver arrangements perform far more admirably given the task. And this is also why THX certification is not given to driver arrangemnts of a horizontal mtm (should the manufacturer even bother to apply).

Nowhere did it say the speaker was wired out of phase, wiring was not even mentioned. It stated "the drivers in opposite acoustic polarity to the left/right front speakers." This would also refer to horizontal tm designs, along with the horizontal mtm's in relation to the left right being vertical driver arrangement. But what is amusing is that many speakers will actually have the tweeter wired out-of phase with the lower frequency driver to maintain a smooth frequency response throughout the crossover region depending on crossover chosen. So dependig on whether or not the center speaker purchased was of the same make as the front l/r, one could actually have thier high-frequencies in the center channel out of phase with the left and right.