It is not mandatory for you to pay extra Insurance. It is your choice because car rentals already have full coverage on their rental cars and they just try always to make extra money with the clients. Next time as the attendant to show you what type of insurance they have. Think about it, why would need car rentals company not have full insurance when they giving you the keys to 10 to 40K Dollars car. What would happen if the car gets stolen, are they going to make pay for it when you can just go to the police, filed a report and then flee the country if they try to make you pay for it? It wouldn't make any sense.