Nick, there're two reasons often given(by Dolby and THX among others)for using two rear surround speakers: first, even with a mono signal that should be imaged at the center back(which one speaker would appear to be able to do well)a psychoacoustic phenomenon can cause a "reversal" and the sound supposed to come from directly behind can instead appear to come from directly in front; secondly, using processing such as DPLIIx or Logic 7, two and five channel material can send different "stereo" signals to each of the two back speakers. On the "reversal", it's been suggested that a single very wide dispersion speaker such as the QSs is less likely to be subject to this, but I haven't seen any evidence on the point. On the "stereo" back surrounds point, this would appear to be worthwhile for present two and five channel material as well as possible future material with seven discrete channels.
