achieving the smoothest response across multiple seating locations might not be the top priority for everyone, and that's where full range side speakers can be useful.
Actually, even with full range speakers, the statement is still true. This is simple physics and no speaker has been designed to violate the laws of physics. Once the speakers (and the single seating location) has been established for best imaging/sound stage, the odds you'll have smooth base response at the seating location is virtually nil.
The use of multiple subs, following the Harmon method, will provide the most consistant response across a wide area...it will, at the same time, provide smoother response. Multiple subs will NOT solve all the problems; however, multiple subs will make the process to calibrate the bass easier (across one, or multiple seats). The follow on process requires equalization and room treatments. One area not addressed (yet) in the Welti work (although Todd, Floyd and I have had discussions about this), is the use of phase and SPL adjustments to multiple subs to further reduce the work required of treatments and EQ.
Stereo subs have been shown, time and time again, to provide no "stereo" or added "envelopment". In fact, when these effects have been observed, it has been shown that the purported 'localization' was really the use hearing the mechanical and air slap noises from the sub woofer(s). Once these noises were eliminated, the "stereo" effect disappeared.