Yes the rental companies do that to make extra money but if you put a second driver in the car and there is an accident, the police report will name the driver and your insurance company will be involved. If your second driver is not properly covered under your policy, and even if they are, your insurance company could refuse to pay the claim because that second person wasn't listed on the rental policy as an allowed driver even if the accident wasn't your drivers fault . If your insurance company refuses the claim the car rental company could come back on you as the renter for the liability because you did not disclose to the car company that you were allowing someone else to drive the car.
That's a dangerous game to play with a rental company for the sake of saving a few bucks and it could come back to bite you seriously in the end. Is it really worth the risk?