No one but mappers will ever notice it.
Maybe that is because we expect too much of it? In a normal dark town roofs arent lit either. Why in tf2 is it even needed if you cant walk on it?
Only if you can walk on it, then it needs light to make players aware you can. Without lights players are going to assume you cant walk on it which is exactly what you want in that case.
Where in normal maps details near doors and such make players aware you can go to that area, in dark areas its simply light that does that job since most props are less visible. Even in a bright map using a diffirent light tint for indoor areas shows information.