I've got some comments to add to this, since I've recently added a center channel, using identical drivers and cabinet design, to my theater, for the purpose of having proper 5.1 playback to aid me in evaluating my sound mastering for the DVDs/BDs that I produce.
I have a somewhat unique situation with my "stereo" playback system, in that it's equipped with the Carver Sonic Hologram Generator, an innovation of Bob Carver, circa 1982, which has been imitated, but never successfully duplicated in digital form. With the Carver Hologram, I get a very real center channel, just as if the center channel speaker were active, and I tend to find that the sound is more natural. When movie soundtrack sound eminates, actually, from my center channel, it doesn't strike me as sounding quite as seamlessly-integrated. Dialog on SOME discs seems too loud and even a bit squawky. However, on a classical music concert disc that I authored, the center channel sounded fine and uncolored.
I can safely say that with a phase-coherent setup, a center channel is unnecessary, except if you're serving multiple viewers, in which case only a minority are in the phase-coherent sweet spot. In such cases, the center channel provides better imaging to off-center seats.