I have been an insurance professional for many years. The most common question I get is "should I buy the extra insurance on a rental care". In general, the answer is no. If you have full coverage on your auto insurance - Liability/Comp/Coll - this coverage transfers over to a rental car. So, in essence, you are buying double coverage if you purchase the extra insurance offered by rental car agencies.
I'm not sure if that is a specifically US thing, but in the UK I've found that rental cars always come with insurance for the car alone, not as an optional extra. It's also not part of 'my' insurance but rather the rental company is the one taking out the policy.
