In fairness a light is going to light up quite a bit of the room and CAN disorient your target. That said though; I must have a higher electric bill than most as my house is just never pitch black where I would need a light and hasn't been since my kids were young. Got in the habit back then of providing some light so they could make it back and forth to the bathroom, kitchen etc and its a habit that never died. But honestly even if my house were pitch black I feel like I would still have the advantage. I mean who would be better able to navigate my house in pitch black better than me? Just what could a robber etc do in a pitch black house where he couldn't see a thing? Would he have to have a flashlight; providing ME with a well defined target? I'll grant you in unfamiliar surroundings a light might be a good thing (such as a LEO searching someone elses house) but in your own home; not so much.
I think rail mounted lights are just one of those things that people think they need but if really thought through; they are not really sure why they think they need them. But then, that's why they make chocolate and vanilla; different strokes for different folks.