I remember seeing a big long-winded post by somebody on AVS or HT forum describing the science of it. He may have been blowing hot air, but he did say that studies have been done (though they were older - back to the to the 70/80s) that concluded that ears don't process sound the same way from the rear as they do from the front (that evolution thing). According to the 'experts' 2 rear speakers (spread apart a bit) created a more effective sound field. At least that is an extremely condensed version, and I don't recall the exact argument for it. So take it for what it's worth.
