As an example, my NZ bank issued credit card insurance policy has the following:
Section 8: Rental Vehicle Collision Damage and Theft Excess covers the excess you become legally liable to pay in the event of a claim against your rental vehicles insurance policy. It does not cover the vehicle itself. You must take out all comprehensive insurance and waivers offered by the rental organisation, whether discretionary or mandatory, available to you when hiring the vehicle to be eligible for this policy benefit. Please refer to the policy wording for a full description of cover.
So a 5 day, $300 rental would become $500 plus due to having to add all the options like reduced excess. Which makes it rather pointless to have.