I think it's to do with trying to make people realise that political correctness and euphemisms won't make things go away. When a person is gibbering "not that there's anything wrong with, you know, it doesn't matter to me, the fact that you're, um..." then it doesn't matter whether their next words are the sanitised version (a mental health services user, a consenting homosexual adult, a differently-abled person, mature and experienced) or the "call a spade a spade" version (mad, gay, disabled, old). The attitude of the person is still the same. Actually, for mental health in particular, I remember a few years back a bunch of people with varying problems who all attended a local mental health daycare centre who got quite agitated because even the professionals kept changing guidelines on what term to use to describe them - service users, visitors, patients, attendees, group members and so on, it was just confusing.