Actually, I don't think it's useful for that case specifically. My example would be switching between say reddit (very bright UI in default or common subreddit themes) and my IDEs (using white text over dark background) or imgur. The brightness increase is generally not noticeable, but the darkening of bright screens is absolutely obnoxious to me.

The only time I change my brightness is when my laptop is unplugged, or when I'm using it very late at night with no external light.

Others may prefer the Lumen way, but I wouldn't use it even if I were paid for.