Having the work inspected still gives you no gaurantees of quality work. Most inspectors don't know how to size ductwork or systems. Nor do they care about quality. It's all about meeting minimum specifications to them. I had one onspector tell me it was OK to run #10 wire on a 40 amp breaker.
But then, how many installations have you seen that wouldn't even meet minimum specifications? Sure, there's no silver bullet, but it doesn't hurt to have another pair of eyes looking at the work, that's answerable to you the taxpayer, not working for the contractor, and to have the contractor know that other pair of eyes will be along at the end.
It might be in a small town there's only one inspector and all the contractors have learned that he thinks 10 AWG is ok for 40 amps, but they might still not want to take that risk; some week the inspector could come back from a seminar knowing more than he did.