Had to do a little research on this, not having heard of it at all. Appears to be a precursor or pre-derivative to DS/DPL, where the difference between the front R/L speakers are fed to the rears. Not sure how this will sound in movies (I did like DPL encoding when it was the rage), but my preference for music is to eschew any surround.
It's probably not "popular" due to the prevalence of discrete surround formats, I imagine.