This to me clarifies why I feel that 3-4 way floor-stander speakers, which are often produce bass to 40Hz or below, are better in creating a stereo soundstage then stand-mounted speakers in a 2.1 configuration. In my view, the latter is a cost-effective solution, hence I expected it to be inferior. If you want to recover the missing bass response, you have to use two subwoofers, each next to one of the stand-mounters (2.2 configuration) where the low frequency constrained woofer of the stand-mounter is augmented with a subwoofer. However, as you now use more floor space than a pair of floor-standers, why bother? Besides, you are still constrained with positioning the subwoofers as you would with the floor-standers.
Subwoofers were invented to reproduce effects in cinemas. The channel that feeds them are called LFE: Low Frequency Effect channel. It was never intended to be used in music. Audio (as in Hi-Fi) industry have high-jacked them in order to sell cheaper book-shelf/stand-mount speakers first then subwoofers as an extra. They are simply tools to extend sales.
Hi, I would suggest some theoretical advantages beside profits, as you outline above.
Subwoofers are typically positioned on the floor, so lower than typical bass drivers on many floor-standing speakers. As such, they may possibly avoid or ameliorate one SBIR effect. Roy Allison's designs often had the bass driver positioned close to one (or more) boundaries for that reason. Significance of floor bounce has been debated before already (my own personal feeling is that typical microphone positioning for various sources may not "accurately" capture what one would experience in terms of floor bounce if sitting at realistic listening position relative to the actual source in the actual venue, based on what I've seen).
Careful positioning of bass sources could also result in mode cancellation or even some degree of correction, a la Welti, Geddes, Fazendi, etc.
They can also be positioned further away from the satellite speakers, so if more laterally, could potentially benefit from Blumlein aka stereo shuffling effects, if in stereo.
Furthermore, if positioned along lateral walls, could possibly enhance perception of envelopment, a la Griesinger, etc.
Positioning constraints are potentially different than with floorstanding speakers, as i outline above.
Hence, I regard it as on oversimplification to say "simply tools to extend sales," likely depends on multiple factors in personal preference.