I both bike and drive a car. When I'm in my car - despite being a regular biker - I've never felt like I missed seeing a biker with a rear blinky mounted at seatpost level that was blinking. Obviously, one could argue "you didn't notice because you didn't see them, but I just haven't one appear out of nowhere.
But *several* times I've had a biker "come out of nowhere" who had a helmet mounted tail light but no seat-level tail light.
The thing is -
1. That when I'm driving my car, my brain looks for lights at tail and headlight level. A helmet mounted tail light isn't at the right level, and sometimes gets filtered out as non-car visual noise that my brain doesn't pay attention to.
2. A helmet mounted tail light *only* points backwards when you're looking straight forwards. Look to the side, the light is pointing off the the side (and the opposite side of where you're looking). Look behind you - your tail light isn't point behind you whatsoever any more.
I hadn't thought of #2 until I was in the middle of writing this post, but I think that's the most important reason to mount the tail light on your bike - if you mount it on your helmet, it stops pointing backwards when you move your head to look to the sides or behind you.