William,
I had thought that pre-spray agitation was likely to be the reason but surely a brush is more appropriate since not only will it give better pile penetration, breaking up impacted soil at pile-base but, not being absorbent, will agitate without removal.
Examine the purposes of pre-spray agitation:
1] To homogenise treatment - here absorbancy is dissadvantageous.
2] Mechanical disturbance - the physical loosening of insoluble particles and the mixing of soluble elements of the soil with the solvent, dispersant, degredant or emulsifier being employed. The advantages of bristles are obvious here.
3] Physical pre-conditioning of pile to enable more efficient extraction - the 'flattening' effect of the bonnet does not achieve this.
I can accept that in rare instances, agitation by bonnet will have some advantages but not in the general run of cleans
Absorbent agitation on upholstery is a different matter due to the relative thickness of the fabric, the nature of the soiling and the relative absorbancy of the substrate.
I am not saying that what you are doing is wrong, only that there is a better way that requires no more time and no additional effort on your part.
Regards,
John.