Just had a furnace installed, the contractor steered me away from duct cleaning and vari speed blower motors as they considered them a waste of money - the crew hustled hard, and my cost was a value
it seems like a field with higher than normal average of having a bad experience with (thats not a scientific fact i admit, i'm basing that on my experience)
i think the car-salesman analogy is correct, because car salesmen often live up to the stereotype (not a PC thing to say, but based on my experience)
its based on greed, which is fundamental part of human nature
greed can spur the development of profound things
but i don't think any of the contractors i know are involved in any real profound experiences except the occasional drug addict!