actually, they do. else they would not be derogatory. now, a mental handicap IS a functional disadvantage, although it has no bearing on one's inherent worth as a person, which is where the analogies do differ, as an accusation of idiocy does refer to something that represents functional inferiority in many circumstances and tasks, but it can still be offensive to the people that fall into that category, since the one using the term is using it derogatorily.