If I had to, I'd use a helmet mounted light, but I prefer not to as it might compromise crashworthiness of the helmet via a more concentrated load area, and the increased size and ruggedness of the higher powered and larger batteried lights now. Outdoor and climbing lights that are small (3X AAA cells) or have larger batteries (4X AA or similar size lithium battery) in a flat pack on the back of the helmet and only the small light projector on the front, I think would be safer in a crash.

Former F1 driver Michael Schumacher sustained devastating head injuries while skiing and hitting a rock, and with helmet. (11 years later he still cannot walk and struggles with communication.) There have been statements that he had a camera on the helmet and suspicians that this worsened the injury. Typical helmet cam mounts are designed to easily break away in crashes (thus are often tethered to prevent inadvertant loss), but a direct hit to the top of the cam would still drive it into the helmet.