I think this actually might help in heavy fog. A driver coming up behind you will probably think, "this person is on the brakes, I better not follow too close."I think you missed the point. Its not about when the brakes are actually applied, its about looking like one brake light is always on for some reason. That reason is fairly commonly a short. The worst case scenario for a single brake light on all the time would be someone who is already a moron for having only 1 working light left of the 3 they should, and they're riding it all the time. I see lots more of the former than the latter. In Oregon we can get a great deal of fog, so I see the value of having rear fogs.
Having driven in heavy fog for many years, brake lights (or what might be perceived as brake lights) are one of the most effective ways of keeping people off your tail.