This issue always raises my cackles. As the son of a long time insurance company manager, I've been well indoctrinated.
Car rental agencies cannot sell insurance. Period. Only liscensed insurance agents sell insurance. That guy behind the counter isn't one. What they sell you are basically riders to cover things like your deductible. If you really wanted, your real insurance agent could probably sell you something similar for less.
They want to sell you this, because it's so overpriced for the "protection" it offers that it's basically pure profit.
Do you think that car rental companies do not insure their vehicles? Do you really think that anything you could buy from them would prevent your insurance from getting hit if you caused an accident?
Even for those without their own car insurance, they might want to check with an agent before signing up.
But if you have insurance, there is no reason to take their minimal coverage. Your credit card might give you the same coverage fr free, in fact.
Should you carry your insurance card? YES! Even if the car company doesn't ask you to produce it (most do not), you'll want it if you get in an accident. Also, make sure you carry your rental agreement with you. This serves as proof of insurance and registration if stopped by the police.
Please save your money. If you must, up your policy to no deductible before you leave and chanmge it back when you return.