It is often said of diagnosing people that it is "putting them into boxes"; "labelling them", "pigeonholing them", and that doing it cannot account for the richness and uniqueness of personal experience. I call this "The Argument from Uniqueness".
I would make a single modification to the argument, while nonetheless agreeing with it: diagnosis does not actually prevent us from acknowledging a person's uniqueness or from getting to the roots of this particular person's problem at this particular time. However, it can lead to an excessive focus on the purported "disorder" at the expense of other aspects of their psychology, and this needs to be guarded against. The Argument from Uniqueness then is correct, but its correctness only gets us so far.
It is unclear to me, in the first place, why diagnosis is supposed to be doing the work of informing clinicians about people's individuality when there exist other sources of information (like psychological formulation) to do the job for us. Somehow, in the field of physical healthcare, practitioners manage an inclusive approach where diagnosis and more detailed information are considered important. In mental health they are considered to be mutually exclusive.
Anti-nosologists reject diagnosis per se and are thereby implicitly advocating a kind of starting-from-scratch with every individual they meet. If you are perfectly empathic and omniscient, this kind of building from the ground up would work. Unfortunately, the rest of us need to draw on the hard won lessons of other people's research and clinical insights if we are to be clinically effective. People (with problems and without) are often hard to understand, behaving in seemingly counter-intuitive ways, and responding to situations with emotions different, or differently expressed, from our own.
I will fall back on the illuminatingly controversial case of "Borderline Personality Disorder". So called "BPD" is a misnomer. It's less a disorder than an understandable response to complex childhood trauma. The appropriate treatment is systemtic and empathetic psychotherapy. I don't like the idea that it is called "Borderline Personality Disorder", which is essentially a comment about your entire personality and thus an insult, but notice, even with the scare quotes, I am referring to an "it", a nameless concept that we assume exists in some abstract way. This something is the referent when even progressive mental health practitioners and activists want to talk about the misdiagnosis and mistreatment of a particular group of people. It is correctly asserted that "it is a trauma reaction", but unless there is something to fill the place of that "it" in that sentence, this statement is de-facto meaningless. What's a trauma reaction? Any given case of whatever it is, as far as a screening clinician is concerned, could be a biological disorder, a psychological problem or a simple instance of malingering.
Developments like DBT and MBT have targeted the particular sorts of difficulties that emerge in cases of this sort of problem, and it has opened the field to an understanding and optimism that was sorely lacking before (and still is in some quarters). However, unless we are able to talk about the issue to which these developments apply, its benefits cannot be felt. How, without something resembling diagnosis, can a GP pick out people who belong to the group we used to call "Borderline" and refer them to an appropriate treatment? How, without diagnosis, can we ensure that the victims of child abuse aren't just bunged on Benzodiazepines and told there's nothing else available?
It is never acknowledged that the anti-nosology case itself relies on the essential epistemological short cuts that are currently clumsily provided by a diagnosis. We don't need DSM categories, but we do need some way of talking about mechanisms, aetiology and approaches to help that are likely to be useful. Perhaps this situation has some role in explaining why diagnostic systems like DSM persist even in the face of overwhelming resistance. Whatever the industrial/financial forces that feed the chemical-imbalance/drug* model, the underlying concept of a diagnosable problem (as opposed to a biological or medical one) is not only hard to shake, but apparently impossible to do without.
*I have adapted this sentence to better reflect what I intended to say. It used to read "whatever the cynical political trends that feed the medical model"