In most areas, it's a sellers' market right now, so I don't think a home inspection gives much price leverage, regardless of all but the most dire of defects found.
For the home inspectors; my experience is an absolute mixed bag for competence, some guys are thorough, realistic with price estimates and put things in their proper perspective. Others just poke around with a screwdriver, looking for the usual suspects, but miss many things they shouldn't have or list defects that are out-and-out demonstratively incorrect.
As a buyer I feel I have the expertise to diagnose most issues, but still, they're places I don't want to go (crawl space, roof) and areas of speciality I'm not expert in.
I think in the end the problem is there's alot of money in home inspection services, everyone knows this and wants a piece of the action (where else can a dude with a flashlight make $400 - $600 in a couple hours?) Home buyers are encourage (pushed, often) into paying for one, and either rightly or wrongly it's presented as a cost that will pay for itself in lowered bids.