I do not care to speculate as to why so many are MTM configurations. However, it does not serve any technical benefit over a MT system other than allowing use of two smaller diameter drivers to produce the same SPL as a larger diameter driver. Use of two smaller drivers, of course, allows for the sleeker looking cabinet, as opposed to a more boxed style that would occur using a larger driver. The MTM is actually technically inferior so far as dispersion. In the plane that the two drivers exist aligned with each other, the distance between two radiation sources becomes signifcant in relation to the upper bandwidth wavelengths(in air) of these transducers. Result is destructive interference interaction as the two drivers begin to operate out of phase with each other as the vector angle is increased off axis. The result, basicly, is a more limited horiztonal dispersion when the speaker is set so that the two drivers align in the horiztonal plane.
-Chris
