Mark, with that argument, headphones might as well all be mono.
You can't ignore how far the listener is away from the speakers.
Not really. With headphone listening the right and left channels are fed to the ears, with no interaction of the room, or the ability of one transducer to modify the other.
Now as you move closer to the speakers, speaker listening becomes more and more akin to headphone listening, as you get more and more direct sound and less dispersed and reflected sound.
Now a car environment and especially sitting at a computer desk with speakers spaced four feet apart, is very much akin to headphone listening.
Once you start moving back six to eight feet though you are away from the near filed.
Now we have speaker to speaker interaction and the ambient field to contend with. Think of a boom box, unless you are right close in front of it, it is mono.
The problem of speaker interaction is acute. As the speakers move closer together, then the wavelengths of reinforcement and cancellation move progressively into much more sensitive territory.
Then if you add a center speaker the problems are magnified greatly. This is really acute if the lobing patterns of mains and center are 90 degrees apart.
So if you have speakers spaced six feet apart, then centers and mains have to be 3 ft. apart. A frequency of 375 Hz has a wavelength of three feet, so you have created a bad situation for the distant field. You can make a really good case for not using a center if you can't get mains and center more than four feet apart. This probably is a factor, in the numerous reports on these forums about speech clarity and having to run centers hot. Interference patterns in the 375 Hz region are well into the speech discrimination region. At four feet, and 275 Hz you are still pushing your luck.