I will assume you are referring to its use regarding a person. Family members of someone who is mentally retarded often do not want their loved one referred to as retarded because of the stigma associated with the word. I read a lot of medical charts and the term is used by doctors; they often opt to abbreviate "MR" instead of writing mentally retarded, but they abbreviate almost everything. I think in other places like the media they say mentally challenged which I guess is the politically correct way.