The DSM is not about diagnosis, but some retards use it that way. The purpose of the DSM is being a psychiatric dictionary. That means if psychiatrist talk about [illness xyz] then they are reffering to the collection of symption [a, b, c, d].
And yes we needed the DSM, but it has become irrelevant. You see; one can research the brain of a mouse; just scrap this and that away and see what happens. With humans on the other hand, this can't be done in vivo ("while alive"). So people started to gather common collection of symptoms so they could put a label on a person with these symptoms, and ask the 'patient' permission to study the structure of their brain after death.
Now that brain research has vastly improved, all kinds of overlaps start to appear with common 'components'. Say a person with ADHD has RDS and DAT ("dope transport from here to there in brain") problems.
Now that the mesolymbic pathway has been found not to be a pleasure centre, but more of a 'conscious intersection' of the brain, using a serotonine feedback loop to modulate dopaminergic release, all these 'illnesses' start to make a lot of sense. Dopamine actualy sort of highlights/projects emotionaly charged 'thoughts' into human consciousness.
Now all of a sudden check what effects PTSD has on hormone release. Suddenly bipolar disorder, schizophrenia, hallunications, paranoia, anxiety, autism, PDD, psychosocial all make sense. An example:
Parents neglect child.
Child doesn't learn coping mechanisms.
No coping mechanisms for strong emotional experiences result in trauma.
Trauma means; brain cannot process said experience, resulting in PTSD.
PTSD effects are less serotonine (depression), thus less oxytocine(autism, thus PDD thus psychosocial leads to "schizophrenia), less dopamine (looks like ADHD, but isn't), more cortisol (stress, self-consciousness skyrockets, anxiety) and more estrogen (hello male, here comes your body dismorphic disorder).
And so on and so forth...