Also, the average person likely drives much more than rides a bicycle, so they're much more likely to be injured in a car accident. But no one would suggest mandatory helmet use for drivers, even though that would likely prevent many times more deaths and injuries than mandatory bicycle helmets.
Can you provide some evidence that helmets provide additional benefits when used in combination with airbags and seatbelts? Specifically actual testing. You would probably want to look at auto racing. My gut feeling is that even a slight decrease in visibility in a car wherein visibility is already limited would make a collision more likely and airbags would already provide substantial cushioning for your head so long as the seatbelt keeps the driver in the car.
