I'm with John, Ken and the others on this one.
The majority of the total time I spend cleaning a carpet or suite is spent on what I would call preparation, ie, Vaccing, pre-spraying and agitating the pre-spray, it makes the final extraction rinse so much easier and results in a cleaner carpet that dries more quickly as you use much less fluid of all descriptions.
Vaccing, pre-spraying and thorough agitation have even resulted in customers thinking that the job is finished when they have come into the room at this stage.
Just to bring it back to Pauls comment re: painters, do you think a good decorator would just slap a new coat of paint onto a dusty, grimey, smoke stained, wall? Of course not, he'd sugar soap it, fill and sand it smooth as required, which would probably take longer than putting the new paint on afterwards.
The better the prep, the better the finish.
Regards
Mark