Three identical front speakers is the most important so that imaging, soundstage, and panning stays uniform across the front, where we are most focused on such details. The only reason so many designers use a differing center is for typical home theaters where the speaker would visually obscure the display. [They want the speaker's perceived "acoustical center" to tightly cling to as close to the correct location, the center of the image, as it possibly can, yet not block the view.] The wide oblong box that is often seen is a vestigial remnant from the days when they needed a shape for the platform they typically rested on, the top front edge of a CRT TV.
In real movie theaters all three fronts are the exact same model speaker, at the exact same center-line height, and hidden from view behind the acoustically transparent screen. As a car, or whatever, moves from the left side of the screen to the right, it correctly has the sound evenly panned from L, to center, to R, without sonically arcing over the top of the display or dipping under it, due to a less than ideally placed center. The whole point is to make the visual experience of localizing an object, one sees, match the sonic experience of where it seems to acoustically emanate from.
When Dolby Digital was introduced, then called AC3, the current trend to use dipole speakers for the 2 surrounds was changed to a recommendation that all probably should be the same as the fronts, since all control was now done electrically instead of acoustically.
The original THX spec actually required that all 5 speakers be the exact same speaker, but due to marketing pressures they were pressured by retailers and customers to allow for inwalls, differing centers, dipoles for the surrounds since they were common, etc.
Edited by m. zillch - 3/28/14 at 5:47am