I've only had a couple rentals in my life; but, the way I understand it is if your car is out of commission and your driving a rental than your insurance covers the rental as though it were your own car. If, on the other hand, you are simply renting a car out of convenience and your car is roadworthy, than you must purchase additional insurance at the rental company to cover it.

I've done both of these examples as I rented a car while my Dodge Daytona I.R.O.C. RT was in the shop getting a new head installed and my rental was covered by my insurance. Then, I rented a car to drive to York, PA since I had just bought my '96 Dodge Ram new and didn't want to rack up 800 miles round trip on it so I had to purchase insurance at the rental facility to cover the rental.