Victor Mike - the definition of freezing fog I am familiar with is " a visible aggregate of predominantly supercooled water droplets which do not fall to the ground. The visibility must reduce to 1000 metres or less and the air temperature must be below 0 degrees Celsius."
As I am sure you know the danger of freezing fog is that the supercooled water droplets tend to freeze on impact with surfaces and create a significant risk of airframe icing. The reason it is mentioned as a separately defined element in METARS, ATIS etc is to highlight this danger to aircrew.