I never use them on the trail, myself. No need. Nothing comes up from behind. I do use my 8 billion candlepower headlamp though. The trail is unlit, wooded, and I've seen all manner of creatures (human and otherwise) lurking on it after dark.

I turn it away from oncoming humans unless they're unlit or unreflected, in which case they get full coverage so they can't surprise me. It amazes me, the people out with zero lights or reflective gear. What happens when two of them meet head-on? Especially cyclists?