Suppose that you only have liability and comprehensive car insurance and you allow your roommate (who doesn't have car insurance) to drive your car to run an errand. What will likely happen if there is a single-car accident causing moderate damage to your car while your roommate is driving?
Then the good idea would have been to add your roommate on your insurance temporarily, while he/she drove the car. Since they got into the accident, and don't have insurance, then they're liable.
