I believe that the surround channel in dolby prologic is not L-R but rather anything that is in both L and R but is 90 degrees out of phase between the two channels.

Running regular 2-channel audio through dolby prologic often causes a "collapse" to the centre channel effect. By sending L-R to the centre and also sending the full L and R channels out you do get an "expanded" feeling of soundstage. Denon receivers with 5-channel and 7-channel stereo modes do exactly that and it sounds pretty good for casual listening seeming to "fill" the room pretty well.