While I only tested it on my Surface Pro 2, I haven't seen any battery life difference.
In real world, I GAINED battery life, as I used to increase my screen brightness as it dimmed the colors on dark screen to compensate.
I bet you the feature was supposed to work on reverse, but it one of those Intel bugs that will never be fixed.
Where on dark screen, it will boost colors and increase contrast so that you don't need to increase the screen brightness. Now, THAT would make sense. I think it would still be annoying, but at least it would make sense. This is what Nintendo does on its DS and 3DS game console when you turn one the power saving feature. Well it does it a bit differently, which is more cleaver, and less visible to the eye in a gaming environment.