I take a headlamp and a keychain LED light as backup. I have been out with a group that on a final day out hiked so slow we ended up walking in the dark - I could also see myself hurting a leg or hip and being extremely slow, and walking out after dark in the trees becomes an exercise I wouldn't want to do without a light of some kind. Starlight and moonlight are fine some of the time; other times, you want to find that trail, but it's overcast or the trail goes deep into acres of conifers.

A light also allows me to shine it on the bear who's licking the bear canister, so I can arrange to not hit him with the rocks I'm throwing.