The extra light could be helpful, in theory, but it would hamper the people using it more than they would realize. The room was filled with fog, not steam, and the light would make it very difficult to see through. I still expected the lever to be pulled, simply as a natural response to, well, a lever existing. Most people will pull an unknown lever or push a random button, just to see what happens. Even if they knew what it did, there was a good chance of it being used. Humans prefer light, even if they had been in a dark dungeon all day and it would destroy their night vision.
