The majority of the terms which were once medical but now pejorative fell out of medical use because they no longer reflected current scientific thinking.
So 'mongoloid' was a descriptive term originally based on the observation that many people with DS have an epicanthic fold in their upper eyelid. It fell out of use as it became supplanted by the more accurate scientific term 'Downs syndrome'.
Similarly, 'cretin', 'feeble-minded' etc were originally medical attempts to categorise different types of mental retardation, but fell out of use ages ago as they didn't actually reflect any objective categories. In the same way 'spastic' is not a synonym for all types of CP, it's a subtype of CP, and can now really only be used for that specific meaning.
You can see this process very clearly with the use of 'Dumb' to mean somebody who can't speak -- the inability to speak is not actually a single category, there may be loads of reasons why people are unable to speak, so 'dumb' doesn't really have any appropriate scientific usage, whereas terms like 'blind' and 'deaf' are still in use now (albeit being supplanted by the more accurate 'visual/hearing impairment').
I can't get hugely excited about the inadvertent use of non-pc terms by people who don't know any better, but I do think once it has been pointed out you that you've used a word that is arguable offensive or inappropriate it's no more than common sense to try and avoid using it again. I don't think that equates to 'censorship' in the way that Microwave seems to be implying, and I do think it's entirely comparable with the use of a term like 'half-caste' -- even if no offence is intended, people will be offended, so once you know that, you should make an effort to use appropriate terms.