Sorry, I missed that. In my case it has to do with placement and the way my room reacts to low frequencies. With a single sub you are free to move it around your room to find the smoothest frequency response and best sound at the lower frequencies. With a pair of full range fronts, you are limited in positioning and what may be good for bass may not be good for the higher frequencies and imaging. In other words, one speaker may have fantastic bass and the other may have a bunch of peaks and dips that just kill the overall effect.
If you have a room that is low frequency friendly, then you wouldn't need to go with a sub for full range speakers. For music, I think this would be the best case. For movies you might want the extra extension that a sub would give you to hit some of the really low stuff they put in soundtracks.
I'm with you though. If I know I can't get away without having a sub, why pay more for extra drivers that you can't use.