I've always thought the very same thing you mention is so stupid. The word "retarded" actually has a true medical connotation as describes diminished capacity of something. It is a very specific word and was accepted to describe individuals medically. So, it was acceptable back in the day. Then someone thought it sounded too harsh and came up with mentally challenged. Eventually that became too harsh so have to come up with something else. The whole thing is stupid, because it doesn't matter the word, the word will eventually become too harsh and not accepted. What is funny, is that it doesn't have an effect or change the condition, regardless what you call it.