Hi Mtrycrafts,
According to several sources, including this website, low frequency sounds (i.e. below 80 Hz) do contain stereo effects. Not only that, there are problems in having a subwoofer placed too far away from the main speakers. Having two full-range stereo speakers can help to smooth out bass response. Of course you can do this with two subs, but that takes up much more room.
Full-range speakers can be more efficient than a sub + speaker set up, if they are of a horn-loaded design.