It's a pretty crappy headline. My apologies for the length and tangential nature of this post. This is a very personal subject for me.
The problem is that we really don't have a decent understanding of the brain (or its abnormalities) at all. We have collections of symptoms appearing in varying severities with varying results, and we have treatments that alter those symptoms. As far as medicine goes, that's really about it.
The problem with a diagnosis is that it's a label. Someone who says "I'm bipolar" can expect that every action will be judged harshly as to whether it's actually their intended "normal" action, or the manifestation of their depression or mania, whichever happens to be the case that day (or hour). A child who's inattentive in school may just be bored, but the diagnosis of ADD opens the door to differently-structured classes that may help - as well as opening the door to ridicule for being different. Sometimes, yes, it's better to stay undiagnosed, and sometimes it's better to get the diagnosis and do nothing with it.
On the other hand, diagnosis is necessary for any treatment. Someone can understand "I'm sad all the time, and don't like it", but without the term "depression", it's very difficult to find information about how to improve. I've met several people who, in the 90's when depression was highly stigmatized, had traumatic experiences that they couldn't talk about and couldn't do anything to recover from, partly because they wouldn't consider the possibility of actually being "depressed".
To make matters worse, there are still an enormous number of people who simply deny the existence of any mental illness. They assume that kids with ADHD are just being active children, or people with depression are just sad, or people with bipolar disorder are just moody. The illness isn't what's visible from the outside, though. The illness is what's happening in the brain to cause the outward symptoms. The ADHD child can't calm down and focus - his mind always jumps to doing something else. The depressed people can't cheer up - even happy times are often plagued by a sadness that's always present in their minds. The bipolar person can't control their mood - the emotions are overwhelming.
What's happening now, albeit slowly, is that the stigma is being countered by awareness programs. This story is in a similar vein to the one a few days ago decrying DSM-5 for not being valid regarding mental health. As our understanding and openness about mental illness improves, we're starting to recognize that typical Western medicine may have done some serious harm to our society. A recent Broadway musical explored this question well.
In next to normal, a woman who grieved four months for a dead child was diagnosed as "depressed", and began 16 years of treatment. One of the questions explored is whether her illness was really because of the loss, or whether it was because of the trauma of ongoing treatment. There is no answer. There is no happy ending. There's only the promise of a next-to-normal life, where everything is perfect except for when it isn't, and there's always some new treatment to try.
That's the ongoing problem with our current handling of mental illness. We have collections of symptoms, and drugs that treat them, but we don't really understand how. The DSM-5 is so vague and imprecise that a particular symptom is painted with a wide brush to be a whole set of disorders. With no testing for suitability, medications are tried that aren't fully understood, in the hope that it's the right drug to set everything right quickly. When it doesn't work, another regimen is proposed, also with little or no testing for suitability. As the patient's treatment drags on, whole classes of drugs are ruled out for their side effects, then brought back because they were better than alternatives.
What would be better is to avoid diagnosis labels (or use only more specific labels) until we can understand and test for the neurological conditions that lead to the symptoms of mental illness. We could differentiate "depression due to low $HORMONE" from "depression due to physically-damaged $STRUCTURE", and "low levels of $CHEMICAL" from "high levels of $OPPOSITE_CHEMICAL". From there, drug treatment can be centered on solving specific problems, rather than just brutally shoving the mind in a particular direction. Once the underlying neurology is a bit closer to "fixed", therapy can be more effective at helping the patient cope with their chronic condition.
Nobody's perfect. All we can do is help people get a little closer, until they can be satisfied with their next-to-normal lives. The DSM-5 doesn't help that as well as it could, so mental-health professionals are avoiding it.