I'm surprised at how many bogleheads enjoy overpaying.
My car insurance that I purchase at home covers me 365 days a year, in whatever vehicle I drive/rent (NOTE: check your policy language carefully. Mine covers me as long as it's not a TRUCK, VAN, or worth over $65k MSRP) This costs me about $2 a day, with a deductible that I'm comfortable with plus a $1 million combined single limit of insurance, and full tort.
I agree that a credit card offering FREE, primary rental car coverage would be ideal (so as not to involve my insurance), but those seem few and far between. I believe Diners club cards, which are no longer issued, provide that coverage. I drive a rental as carefully (or carelessly, as some would accuse me) as my daily driver. If I had the skills to predict what would happen on what day, I'd spend those insurance dollars on the lottery numbers rather than insurance.
If I'm in an accident that I cause, more than likely my insurance will already find out about it as they monitor my drivers record. If they don't find it on their own, I will be required to disclose it when I call around to compare rates or change coverage levels. There goes most of the benefit of paid car rental insurance.
The car rental companies offer this at a price, so does Amex, because the have studied the statistics and are all making an ridiculous profit margin on it. I've seen many at the rental kiosks hit yes too many times and then their $100 a week rental is now over $500. The look on people's faces when they get the upsell "you wouldn't want to have to put a CLAIM on your insurance, would you?" is amazing.
I've had over 200 successful car rentals. Three cracked windshields, one flat tire, one scratched rim, one broken door handle, three valet-inflicted scratches/dents. I have never been charged once. I agree that you'll hear of intimidation stories online about rental companies charging for loss of daily use. Those disappear rapidly when your own insurance company states they will only pay them with fleet utilization records.
Ask a rental company how many wrecked cars come in, after a days' returns. Probably less than 1 in 2000. Even those paying Amex's reasonable $25 a rental, that's $50,000 in insurance paid out for those $300 bumper repaints or $1000 door replacements.
Note: there is one scenario which I have strongly considered: Most cards indicate their coverage is secondary IF you have primary insurance through other means. If I do not have a personal policy, because all my automobiles are owned by a corporation that I control, then I automatically qualify for their primary insurance benefit. However, since most card benefits only pay for the car itself, not liability caused to others, it' still not a good plan