when i started out cleaning my first job was a deep piled saxony, was full of dog hairs that matched the colour of the carpet (dark brown with black flecks) an ear-ring embedded in the pile, lots of tiny stones in the hall section of the carpet...moral of the story...
"you cant always visibly see the sh*t in the carpet."
yes the customers may well get a good job without pre-vac, but for my own peace of mind, and for the fact that a 2k machine isnt bulletproof, for me pre-vac is an added insurance policy and somethin that customers appreciate.