Well, then I'll admit I don't understand at all. It just doesn't read that way to me at all.
"Some people with autism may not speak, or have fairly limited speech." This is a fact, not an aspiration, surely? Some of us don't.
"They" (the people in the sentence above) "will usually understand what other people say to them,"
Well, most of us (not the people with severe autism, perhaps) do usually understand what other people say to them. Not all of it, but what they're surely trying to say is that you shouldn't assume that the person who's not talking 'isn't there'. A common thing said by those with an ASD is that people talk over the top of us, or about us right in front of us as if we're not there/too stupid to understand what you're saying. I read this as them trying to explain this.
"some people....prefer to use alternative means of communication themselves, such as sign language or visual symbols."
This is true.
" Others will have good language skills, but they may still find it hard to understand the give-and-take nature of conversations, perhaps repeating what the other person has just said (this is known as echolalia) or talking at length about their own interests."
This is also true.
I don't think the NAS can detail everything about autism and language in just that paragraph, which is why the site has endless papers detailing further information?
www.nas.org.uk/nas/jsp/polopoly.jsp?d=1757&a=5319 is an example of such a further information sheet, which clearly shows the difficulties in communication with pages and pages of helpful info.
I don't see how people can take just that one paragraph in isolation and say the NAS is promoting the idea that we shouldn't be encouraged to speak, etc?
I think it's important to remember that one 1 in 5 people on the autism spectrum is a child, and that only 1 in 10 of those on the autism spectrum is a child with severe autism. The NAS have to speak not only for that 1 in 10 (and vital that they do, of course), but for the rest of us too. I think they do a good job, myself. Not a perfect job, but a good one.