I put a flashlight on my helmet for a game in the dark once. It was a game of zombies, so no one would actually be shooting at me.
It was actually really uncomfortable. It only shone where I angled my head, and the rail angled down a bit (to aim where you're walking), so if I wanted to look up ahead, I had to tilt my head back. Having the flashlight on your gun, or even in your hand (as much as it gives up stability on the gun) is a lot more practical in a hot environment than on your helmet.