Steve,
Whilst there will be many who will argue with my oppinion, I consider the "Host"-type method to be suitable only for cosmetic cleaning - an alternative to bonnet buffing.
Whilst it is almost 'idiot-proof' in that it is hard to over-wet any carpet with damp, ground-up corn cobs, I would question the amount of soil removed and how much proteinous mater and detergent residue is left in the carpet.
Useing the correct HWE equipment and chemistry, a skilled opperator will not only remove more soil (and dust mites) from the carpet but also achieve near-comparable drying times.
As to the customer's pre-conceptions, brought about by previous experience of bad cleaning the answer is to put your name and reputation on the line - If you do not have the confidence to make a 'payment on satisfactory conclusion' deal how can the client have confidence in you?