The worse problem is when one wheel is pointing left and the other right. If the only problem was the car not rolling straight the driver could turn the steering wheel, and then it‘s straight. When the wheels are facing in opposite directions the driver can’t correct for this and actually may not be aware of it as it may not cause the car to turn, but with the tires (slightly) sliding to the side instead of rolling straight ahead, they will wear out much faster then they should.