Chris, the way a charge pump device works (between the battery and the LED) the answer might be that the brightness is not affected much by low battery voltage until the battery is almost dead.
Despite the fact that most of us carry a small headlamp for backpacking, and despite the fact that many of us carry a tiny "pinch-light" or coin battery type light for a backup... one of the more effective night hiking tools is your own human night vision. Granted, it is not so perfect on a dark night, but on a moonlit night it may be all you need for an open trail. Whilst in Army Infantry training many decades ago, I was taught how to properly use my night vision. It is a lightweight backup system, has multiple uses, and it weighs nothing at all. You do have to fuel it by eating the occasional carrot.