Your 2 friends who were ejected from the cars and survived must be the rare exceptions. they were lucky enough to not be thrown into oncoming traffic or into a tree/building/ or off a bridge. I dont have any statistics but I would most likely say the safest place to be durring an accident is inside the vehicle.
I always wear my helmet . the chance that it will cause additional harm is pretty low. If it works just one time its worth having. Anyways, you've already bought it! its not like you can return it. And if your wearing it and it fails durring an accident where you suffer sever trauma then atleast you can sue giro!!