A rear fog light is a great safety item and should be on all cars in the USA.
BUT, it's not and outside of urban areas or areas with a propensity for foggy conditions, it's a feature largely misunderstood by the public and by law enforcement. For years, I have heard story after story from people that used rear fog lights about being chased down by people so they could kindly tell them that they had a burned out or somehow malfunctioning light in the rear of their car.
Worse yet is the Police Officer that doesn't know what a rear fog light is so he pulls you over for an equipment issue with your car. Sure, you can explain it to him and probably make it all go away, but what if there is another offense in the mix or if you have recently left a bar or any number of other scenarios. As a driver, we just don't want to give the authorities the pretext or probable cause they need to initiate a traffic stop. From that, I'll say I know an Officer that has pulled over Benz drivers before when they are out late at night and they have pulled that complicated headlight knob around and turned on their rear fog light without even knowing it. He caught a few DUI using that pretext to pull them and talk.
Something to think about. Personally, I think it should be standard safety equipment, but almost everyone you know doesn't have a clue what the extra bright single light in the rear of some cars is.