Too many people think that their auto insurance policy plus their credit card coverage benefits equals all set should i buy auto insurance for rental car. Again - maybe you are, maybe you're not. Have you read the documents? Too many renters learn the hard way that what most cards provide is "secondary" coverage, meant only to bulk up your auto insurance policy, paying out after that has been exhausted. Your Visa card, for example, can be a great asset, but if you don't follow all their rules (the coverage may be invalid if you rent a mid-size car in Cincinnati on a Tuesday when it's raining), the very thing you think is going to keep you out of trouble might end up leaving you in a world of hurt.
Some higher-end Mastercards might be great, for example. Others card levels may offer little protection. Examine each card document carefully. Finally, note that credit cards tend to mostly offer collision, damage and theft coverage, if they offer anything at all - personal liability (hitting a pedestrian, crashing into a store front, or injuring drivers in another car) and personal effects are generally your problem.