Driving last night I came within a second or two of knocking a commuter cyclist off his bike and my immediate excuse would have been "sorry mate I didn't see you"
I had stopped at a 'T' junction on a dark country road and the cyclist came from the right hand side and I didn't see him until he passed the front of the car.
I followed him convinced he didn't have a front light, which was surprising as he had two rear lights and was wearing reflective clothing - not the sort not to have a front light. However as I passed him I immediately thought - no he has no front light. It was only when I pulled away from him and saw the whole of him in the rear mirror did I see that he was wearing a helmet front light.
The guy was quite tall and what was happening was that close to the car (to the side and back) his helmet light was in a blind spot on the car due to its height.
Therefore, I don't think you should rely on a single front helmet light, wear one by all means as a supplement, but have a handlebar height light as your main light. Additionally a helmet light turns with the rider head, if the rider is looking anywhere than dead ahead, there is a risk he won't be seen.