You're confused. "Idiot," "moron," "cretin," "imbecile," and "retarded" were all scientific terms at one point. The problem is that not-so-bright people use the terms in the pejorative, and then they take on a negative context. Like you're arguing we should do, for instance.

You're confused. By the 1960s, "mental retardation" became the preferred medical term, gradually replacing previous diagnostic standards such as "idiot," "imbecile" and "moron" -- terms that had come to carry pejorative connotations. "Retarded" has whatever meaning you apply to it.

Any words that get used as slang or associated with something a large group of people consider a target for insults or abuse invariably fall victim to this cycle of over-protection by people such as yourself, who as well-meaning as you are, you're doing a disservice. In your attempt to show you care, you're making the association between someone who is retarded with being a negative thing and not a simple fact. In 20 years, you'll be fighting to not use the phrase "pervasive developmental delay" as it's considered an insult.

"Nigger" was once socially acceptable. This was used by slave owners, and it gradually changed to "negro". Then, because this is a protected group, well-meaning people like yourself felt they wanted to shed that word in favor 'colored' that went away and turned into 'black'. To become more PC, people pushed for 'African-American'. Then, as that began getting old, and a new generation of non-african-african-americans came along and said, "we're American", and the term 'black' came back into favor as well. Now, it seems that it's okay to use "nigger" again (if you're black) and "nigga" (if you're not black), while both black and African-American both remain PC.

In all of that, I feel comfortable saying someone is 'American' if their nationality is the question, and black (if i'm asked what 'color' they are). I don't cringe when someone uses 'nigga' or 'nigger' or 'African-American'. I wouldn't get upset if they said 'negro' or 'colored', but I might make a mental note of it as it's not common to hear. I try not to make any generalizations, opinions, or blanket stereotypes either positive or negative for any ethnic group and I don't recognize the need to fight for the eradication of a new label every 10 years.

All of that said, I can see this is something you feel strongly about and I have neither a chance of changing your position, nor convincing you that my personal perspective is even acceptable for me to have.

tldr; words have the meaning that you give them, and I'm sad you have such a negative opinion of those persons living with mental retardation.