what's so special about center speaker ?

The very best center speakers are rarely horizontal woofer-tweeter-woofer. THAT is for marketing (low and wide). If you don't need that low wide look, three matching bookshelf speakers across the front would be far better.

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.

Stacked speakers have long been used in sound reinforcement. For example, Bose makes a speaker with 4 drivers in a vertical array. The effect is to 'fan' the sound in a horizontal plane with less sound going toward the ceiling and floor -- great for church PA systems. An MTM (or WTW) will not be as extreme but you'll get some of this effect with sound distributed in a vertical 'fan' with sound falling off to the right and left. This is another way of saying the same thing others have said. This is not ideal. I use three identical two way monitors even though I have a fourth sitting in a closet (at the time I could only buy them in pairs). ACI (www.audioc.com), for example, mentions that two ways are better for the center channel on their web site and sells their Emerald LEs (350ea) and Saphire III LEs (650ea) in singles (though I've never heard these speakers).