>Ask them if they judge their kid is "useless".

They'll probably say no, but their kid is useless socially, economically, etc. Can't have a job, can't help others, can't have friends, can't have a normal life, hell can't even have a life.

>And since when do we judge human beings on their usefulness?

Since always? That's all we do. And this is not a case of someone who's in a wheelchair or deaf, this is someone who can't do anything other than yell and throw things. That's my experience with an autistic person at the "high end" of the spectrum.