Personally, I prefer to refer to myself as an autistic person or an aspie, rather than a person with an autism spectrum disorder/ASD (for one thing, ASD will always mean atrial septal defect to me). I believe that using adjective-first or modified noun language represents how central autism is to my identity. However, if someone were to say "since autistics can't speak for themselves, we're speaking for them" I would not consider them a good advocate for autistic people. But if someone said "people with autism spectrum disorders need to be listened to. They need to be allowed to have control over their lives" I would consider them someone I'd want advocating for me, because they would be willing to listen.
An example of that is this is a song by the band Devo, about a Down Syndrome man. I, personally, do not like the term ?"mongoloid" being used for people with Down Syndrome, but I like this song. An excerpt of the lyrics are:
Mongoloidy was a mongoloid,
and it determined what he could see.
And he wore a hat,
and he had a job.
And he brought home the bacon,
so that no one knew
he was a mongoloid
Although I don't agree with all of this song, I like how they portray him as a capable person, with a happy life. As far as ?I can tell, this is an accurate perception of the lives of adult Down Syndrome people, or at least how it should be.
Personally, in determining what terms I use, a good rule is to try to find out what the people with the condition referred to prefer. For example, I talk about Deaf people rather than people with hearing impairments because I've noticed many Deaf people prefer this terminology. It's impossible to find something all the disabled people agree on, as we are all individuals (some autistics insist on person-first language), however, if you try to fiond what most people prefer, and don't force your terminology on others, you are less likely to hurt someone's feelings when expressing your opinion. And I ask all disability rights activists to try to put less importance on the terminology used, even if it is hard sometimes (I know I find some terms grating). There are a few terms I think should eventually cease to be used, such as "confined to a wheelchair", but it's more important to look at the underlying content.
Idiot, moron and imbecile used to be accepted medical terms for various degrees of developmental delay, idiot being profound, imbecile moderate-severe and moron mild. It is because of how developmentally delayed people were viewed that those became insults. Then they switched to "retarded" to try to loose the negative associations with the older terms, but the associations remained, because changing the terms without changing the views is futile. Now "retard" is also an insult, so some have switched to "mentally challenged", "developmentally delayed" or "cognitively impaired". If the prevailing view of developmentally delayed people doesn't improve, those will soon be insults as well.