If your credit card coverage is secondary, an accident is going to cost you your deductible plus it is likely to affect your future insurance premiums. If your coverage is primary, your credit card handles it all directly with no out-of-pocket cost to you and no ding to your auto policy rates.

I've also rented cars hundreds of times, and I agree that the chance you will run into a problem is minor. That being said, I did once incur some damage to a rental car, and it was nice to make a single phone call to my credit card company and be done with it. Purchasing coverage from the rental car company would have been even easier, but as most people acknowledge, that's a large price to pay for convenience when compounded over many rentals.