I thought this way also -- the risk is low so why get the extra insurance?-- until I got a flat tire and the rental company tried to charge me $500. When I asked for a break-down of the costs they incurred I found a bunch of things added on that had nothing to do with a flat tire. I was told that if I didn't pay they would then file a claim with my insurance company, hence, increasing my rates.

I finagled back and forth for many months with the rental company and finally filed a complaint with the BBB. In the end, I won, but it took a lot of time (6+ months)and patience. I certainly don't want to go through that again so I think I would purchase the extra insurance next time.