True, but it is illegal.
I find a proper rider has his/her head on a swivel and therefore you get dizzy watching the video, whipping all over the place. As for in the event of say a collision your likely NOT going to be looking where the collision is unless it is of you hitting something. Think about it, you see in your mirror a car racing up behind you about to rear end you, are you going to try to get out of the way or bother to look around to catch it on video? I think I will try to avoid being hit. Most times riders or cagers don't see they are about to get hit unless the vehicle is directly in front of them, so again the chances of having your camera pointed in the right spot is minimal. look at crash videos on you tube rarely is the rider with a helmet cam looking at the vehicle approaching from the rear or side, in a collision. Sure they will look AFTER the collision. But the damage to vehicles is just as much "evidence" as to how the collision occurred.
Lastly, helmet cams also tend to capture things like your speedo, which is not an issue if your a responsible rider. But when your doing 180 and if stopped the cops seize the camera and with a warrant, your sunk, you just provided them with the evidence. A "properly" mounted handlebar cam can have virtually no vibration, is never going to capture your speedo, will give a great POV, and will never get you get you pulled over or get loss, due to wind or impact.
Also when I had both rear and front facing cameras mounted on my bike and was sideswiped, (by a road rager), who then claimed I hit them. The cameras didn't capture the actual impact, but the insurance company, (who had listed me as at fault), were able to see that the bike jerked to the side as his vehicle made contact. Upon viewing the footage the insurer deemed me not at fault and him 100% at fault. So I have actual real life experience. Had I been wearing a helmet mounted cam, it likely wouldn't have moved or the other side could have argued it was simply a head movement of mine. So in that case the vehicle mounted camera, was 100% effective, (there was NO damage to my bike and his car had a dented rear drivers door, due to his coming into contact with my bike). So I can attest that a "properly" mounted vehicle camera can be just if not more effective.