But having only one arm is a symptom. It doesn't explain why the person only has one arm.

You say that the diagnosis easily explains behavioral issues. But how? You have the diagnosis because of behavior.

I do understand the point about using tools that others with the same diagnosis can help. That is what I pointed out myself. It seems to be the only valid argument that I have ever heard. But it still isn't a given that what works for one person will also work for another just because they have the same diagnosis.

I don't really see how the example of your mother is an example of someone treating the underlying symptoms without understanding the cause. A diagnosis of Aspergers or autism doesn't say anything about if trying to desensitize someone is valid or not.

The one about the bureaucracy of requiring a diagnosis to get additional help, that is also the result of having diagnosis in the first place.