I will say the diagnosis helps at times when you're advocating. I just went through this. My child was having more behavior problems as the year progressed, but a lot better than last year. I was hearing a lot of words like "defiant" coming from the school.
When I went to the IEP meeting, the first thing I said to everyone (including my child's teacher) was "Is my child defiant, or is this a symptom of his syndrome?" At first there was some defensiveness, but eventually we started talking about triggers and how to correct him and how to handle the anxiety that any of these programs tends to trigger in him.
I guess my point is that how people frame the behaviors in their minds helps them deal with them differently. If the behavior seems defiant, and the teacher has not other context to put it in, the teacher will take the behavior personally and be defensive and upset about it. If the behavior is framed as a symptom of the disorder, the teacher can pull back and see that it is not about them. I admit that some teachers still can't pull back, but then you have the law on your side.
