Its better to get $20 for the car than to let it sit idle for the weekend.
Not always. If it's a manufacturer buyback, then the incentive is to maximize revenue, and hence not let the car sit idle too long.
However, more rentals today are risk vehicles. The rental agency owns the car outright, and has to sell it themselves at auction. One major issue for resale value is mileage. So there arise more situations where some agencies would rather not rent the car.