Left-right sources are not necessarily stereophonic. The point of stereo is to create a "phantom image" where there is no physical sound source. This works only when the entire audience is in the median plane between the stereo sources. The cause of this extreme sensitivity to position is not amplitude differences between the sources, it is the arrival times of the sounds from the stereo sources at your ears. Very small differences in arrival time - on the order of a millisecond (i.e., listener about a foot off the median plane) cause substantial degradation in the "phantom" stereo image.
The point of W/D/W is to create spaciousness of sound, not a "phantom" center image. When there is a physical source of sound in the desired location - the dry speaker in this case - the ability to localize to that source is extremely robust over a wide range of angles and relative distances from each source (the two wet and one dry) to the listener. In order to get a stable center image with an LCR or W/D/W system, all you have to do is send that signal to the center (dry) speaker. You can do the same with signals you want panned hard left or hard right, because there are physical sources in those locations. As for localization to locations other than left, center, and right, you don't do that with LCR or W/D/W. You can very effectively create apparent motion (via crossfading) of the sound source, but the end points of the apparent motion must coincide with physical speaker locations in order to be reliably perceived by the entire audience.
