Yes, I do use the rear fog light on my ML320 occasionally, even in southern CA, when it rains hard or is foggy. I've also used it to help a friend in a following car more easily pick my car out of the traffic.
I use the front fog lights more often, in fog or rain, or to provide better side lighting when driving in neighborhood streets. I wish Mercedes had put in side lights that come on when one uses the direction signal, as the Ford Taurus had.
Chris