I run lights whenever I ride my bike, day or night. I often have more than just one headlight and one headlight. One day, having my lights might save my life. For all I know, it already has. There’s no way of knowing for sure if lights increase my safety, but I will be able to know if they did not help, i.e. if I get hit.

My lights don’t slow me down much. If they are the strap-on type, the strapping on and off slows me down more than the air drag does. I have a couple of bikes with hub dynamos, and they create a little bit of drag, but it’s not very much, and it’s worthwhile for me since I never have to strap on the lights nor charge them. Aerodynamic drag is relevant only when you’re riding very fast, and even then, it isn’t a burden unless you are highly competitive.