Wannabe, I agree. I think a lot of it is the social need for acceptance, not a shred of thought about whether it means the best education or the best long term outcome for the children.
In the case of ASDs,a mainstream school is often the worst of all worlds. A social nightmare with no way for us to be 'included' - any more than putting a totally blind child into an art college where painting in watercolours is the only medium they can use, then hoping they will turn out to be Leonardo da Vinci. Or putting a child who is totally deaf in a music school where all the exams are 'listening to a piece of music and commenting on it" and hoping they'll become Mozart as they 'have to learn how to do it'.
Some with an ASD do cope in mainstream if it's structured enough and if they have the right support. Many endure it in the same way as others might endure a multi-year neverending nightmare.
All the "oh but they have to learn to socialise and cope in the outside world" stuff is often so much nonsense. We cope best by learning to adapt our world, avoiding social groups and finding ways to cut out the total overload of group situations. I'm all for the right therapies to make our lives easier, but the "bung them in the school and let them learn" approach is often madness.
Each child and family needs to be able to make the right choice for that child, not told they have to like being put into a mainstream school with an occasional support worker/TA who's been given (say) a three hour tick-box course in that disability and is them supposedly an 'expert' (arrghh!) (and I know TAs try their best but they have no way to do their job properly with this little knowledge. They're often treated as babysitters, not teachers).
Even the 'experts' seem to struggle to know what they're talking about a lot of the time. I had a bizarre conversation with a man who claimed to be an 'expert' in autistic spectrum disabilities and said he knew far more about it than I did as he had a Certificate of some sort in it and had worked with people on the spectrum. Well, I've lived with it all my life and I'm married to a man with it and I've worked with lots on the spectrum too. Not to mention being an adviser for it. What good is a bloomin certificate if it builds that kind of arrogance? I'd have been more impressed if the advice he was giving wasn't breathtakingly wrong.