Using the same tech which gave us a solid center "dialog" channel for over a decade with few complaints and impressive results is being used to do the same for the rear surround channels when a discrete 5.1 soundtrack is fed into a modern HT processor. Add a little steering logic to generate a convincing left and right rear channel output and you've got a totally immersive full 7.1 experience. It is easier to market 5.1 to lay consumers who have yet to install 7.1 systems while letting the expert users take advantage of the logic in their HT processors.
I love 7.1 from my system when watching 5.1 encoded content. It is completely immersive and convincing to me. True, i'd rather have diwcrete 7.1, but decoded 5.1 to get 7.1 is still friggin' awesome