Tom, you are indeed a scholar, a gentleman, and a great diplomat!!!
What follows is my own personal opinion, based on my impressions of a variety of operations observed over some 50 years of blissful automotive ownership:
I must admit, that in spite of any inherent beneficial properties contained within the various "flush" product containers, the whole "business" of service writers making recommendations concerning the "need to flush" what seems to me to be nearly EVERY fliud filled system in cars that roll through the front door deemed "due" for a fluid change, certainly could give the casual observer (at least in my opinion) the impression of blatant "retailing" on the part of said service writers (who are, of course, getting a percentage of the service bills written!!!). I am sure that with a given amount of abuse, neglect, misuse, or contamination some systems may need a flush as determined on a case-by-case basis...but not to the extent and frequency that I have observed...especially when the recommendation is being made upon arrival as opposed to after examination.
And, to top it all off, just seeing the "flush" company representative collecting container lids (proof of product usage) from the techs and trading them for cash tends to make me feel more than just a little uncomfortable about the whole precess...again, just my opinion based on my own personal observations.
Of course, when an extra step is added to any process, the labor time may possibly increase, and therefore the bill increases, as does the compensation to all those individuals directly involved.
There are parallels in many other areas of life!!!
Having most of the Service Manuals for all years of Vipers...I have yet to find specific recommendations supporting this commonly encountered situation.
Hopefully, some knowledgeable person here can shed more scientific light on this issue, and correct my own impressions if they are indeed wrong. A large series with statistically significant data, the results of which have preferably been confirmed by more than one independent team, and published in a mainstream, peer reviewed scientific journal would be helpful.
Thanks in advance!!