Hi Bryan
When I first started out in this business quite a few years ago, I recall being told on an NCCA training course that if you rub and scrub at a carpet particularly under wet conditions, then you may often cause distortion to the pile fibres of a carpet.
Even in stain removal we are taught by the professional trainers and on any spotting reference guide 'Blot, do not rub or scrub'!
I look at things like this :
No seriously, I personally have reservations about useing rotary shampoo scrubbing systems on carpet fibres, due to the aggressive nature and tork placed on the fibres. This can and often does distort fibres leaving swirl marks in its wake, which can be difficult sometimes impossible to brush out completely.
Now there are always exceptions to the rule, and in some situations having this system on the right type of carpet when dealing with heavy soiling can be an excellent way to break up soiling prior to extraction (duel process clean).
I personally like the use of the Host type contra rotating brushes because there is little or no tork on the fibres. The fibres are being agitated reasonably uniformly, and are being brushed in an upward motion, not swirling around. This upward agitation will effectively lift the fibres whilst not serving to break them up by twist action under a heavy weight.
My personal view is that the dry carpet cleaning brush systems are an essential piece of kit for aggitation of main traffic areas, to work in prespray, and of course their original purpose to dry clean carpets that are problematic to wet cleaning.
Thats my thoughts on the matter any way.
Regards
Robert