In my case, a diagnosis would cost money when our budget is tight. I already have my coping mechanisms so it wouldn't help me. My son's getting his help so a diagnosis for me wouldn't help him. I'm comfortable knowing that I have Asperger's without having the diagnosis.
At this point, a diagnosis would mainly be to show other people to quiet them up if they claim I'm just making it up. Quite honestly, there is a limited group of people whose opinions I care about. If $RANDOM_PERSON or even $RANDOM_FAMILY_MEMBER refuses to believe me until I get a diagnosis, that's their problem.
I do know others who have gotten the diagnosis in adulthood and more power to them. For me, though, unless I don't have to spend a lot of money on it (or unless money isn't that tight anymore), it's not going to be a priority for me.