Guy's. We are, are we not, offering a service. The customer (remember them) is the person who will decide if the service we offer is acceptable. The majority of my customers are women. The first hurdle is getting through the door to do the audit and explain what and how we will do the job. Let's face it, it is the lady of the house who will decide who will get the job. How many, and there will be some, go to visit wearing a grubby pair of shoe's with a ciggy hanging out of your mouth?
These "ladies" know how to clean. If you don't agree I challenge you to tell your next customer that she hasn't got a clue; will you get the job? As professionals we will have our own view on the rights or wrongs of pre-vacuuming. Ultimately, so long as the lady is satisfied her "nest" has been cleaned to her standard, will we have done our job. The value of that job will be decided by the customer.
Regarding scientific evidence, if proved or disproved, will we take the report along to show the customer, I think not; just as we don't show one detergent against another. The customer assumes we know what we are doing.