I feel very split on this.
My DS has a diagnosis, he is aged 7, and this was as a result of the school pointing out his differences. We paid privately for his diagnosis, but the psychologist pointed out that whilst he met the criteria from the home screener questionnaire, he didn't actually meet the criteria from the school questionnaire. Had his diagnosis been through the NHS or CAHMS, he would have been dismissed at that point. He also doesn't meet any of Criteria 2 of the DSM-5, but because he has 'sensory seeking behaviour', he got the diagnosis. A few years ago he would not have been diagnosed. He also has suspected ADHD, which could account for the sensory seeking behaviour.
I got the diagnosis mainly for the future, just in case. He is doing really well. Very clever, well liked, we support him as a family.
School, despite being the ones who suggested getting a diagnosis, haven't offered any extra support or funding. They've said that he isn't eligible because he is happy and doing well in school. They won't support an application for an EHCP, as they claim he doesn't need it. Interestingly, they also started attaching autistic characteristics to my son which he doesn't have, such as writing in a report that he 'has troubles with transitions' and it took us pointing out that he doesn't, for them to acknowledge this. One of my DS's strengths is his flexibility and spontaneity. People do tend to stereotype autism, and because my son does not fit a lot of the classic autistic characteristics, I really fear he will be stereotyped, which has already started happening.
Diagnosis doesn't = support. Being the type of parent who is constantly in contact with the school, LA and has the money and resources required does. Having worked in a school, those parents are also the ones that get criticised and disliked by all the teachers!