Regarding courtesy cars in the US, it's important to remember if you are a foreign visitor that you almost certainly are not insured.
Problem is, the grizzly old FBO guy will certainly assure that the car is insured. It may be, but unfortunately he doesn't appreciate that because you don't have US auto insurance on your own car, you are not! So if something happens, the car may be covered but any 3rd party issues will be yours alone.
The only way to fix this that I know of is an American Express travel policy that covers rental cars in the US. I've tried extensively to get UK cover extended to the US but UK insurers won't go near it. If someone knows better, please advise.
I've used these cars (a wonderful system) extensively at my own risk but my view on insurance after 30 years of unblemished driving in the US changed somewhat when I had a mishap in one of Mr Hertz's finest (and totally insured) rentals. It can happen to you!