Keep in mind that a diagnosis is neccesary in order to receive social security. That means that regularly a psychiatrist, psychologist will label a person that falls short of a DSM IV category, but that can clearly benefit from treatment (whether that be behavioural therapy, drugs, ECT, etc). Without that label you receive no compensation for treatment.
Many disorders didn't exist pre-19/20th century in terms of medical description, or were chucked in groups as 'hysterical' or 'insane'. That doesn't mean they're merely figments of 20th century society. There were times when certain diagnoses like ADD were bandied about like ping pong balls.
Much more worrysome are the attempts of the pharmaceutical lobby to stretch the inclusion criteria for disorders, in order to create a greater market for their drugs.