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).

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.

MTM's do have several advantages if properly constructed. One would have to do some very serious studying of the drivers, their dispersion patterns, optimum crossover frequencies, the spacing between drivers, placement on the baffle, etc... before building a marketable model. then again, this is true of ANY speaker design.

The reason that the horizontal MTM design is so widely used in the HT market is for the very reason that Ezmeralda and Danner "bag" on them. It has limited horizontal dispersion characteristics. Since you have a speaker on each side of it ( the mains in an ht system ), they are trying to limit side to side dispersion and keep the voices centered. Not only does this improve imaging and localization in this case, it also minimizes lobing and interference with the two other front speakers.

Some of the other benefits are that the designers can use multiple drivers for increased surface area, greater power handling and low frequency extension, lower distortion due to reduced excursion, smaller drivers for improved transient response, etc...

Obviously, another consideration would be cosmetics, as this design is typically less intrusive than a vertical array would be. This type of speaker will work best if mounted on top of the TV with a slight downward angle ( to minimize ceiling bounce ).

I think that Vance tends to look at ( and comment on ) these designs as if they were running them as "stereo" speakers rather than "special application" speakers. I agree that the potential for problems are much greater with a design of this type, especially when mounted horizontally. When properly executed and the layout of the room is taken into consideration, they are hard to beat for this type of situation. After all, most people try to watch a tv / movie while viewing the screen as centered as possible. Unfortunately, most of these designs and "home theaters" are not properly executed, so consumers end up with a lot lower performance than their systems are capable of. Like anything else, you have to buy what will work best for your specific situation. Sean
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