I don't think anyone in the field seriously claims that DSM-5 diagnoses have some particular validity. Most of them are just clusters of highly correlated symptoms without a clear, objective causal link to any underlying pathology. But as a practical matter, in clinical practice it's useful to have labels for those clusters of symptoms. This helps to facilitate communications between providers, as well as in generating the documentation required by payers to justify insurance claims.
> I don't think anyone in the field seriously claims that DSM-5 diagnoses have some particular validity
In my personal experience, many clinicians – and even researchers – talk about diagnoses in a reified way which only makes sense if one presumes they do have some sort of validity. Many of them also appear to engage in the "motte-and-bailey fallacy" – talking about diagnoses as if they were valid, falling back on "we all know they aren't really" whenever that talk is challenged, and then going straight back to talking that way as soon as the challenge has departed.
If one takes the lack of validity of DSM-5 diagnoses seriously, that has certain consequences for research programming and study design – that one ought to prefer trans-diagnostic studies to those focusing on single diagnoses, that one ought to study cohorts which include subclinical cases along with those with a clinical diagnosis, etc – and yet, very many researchers continue to ignore all those recommendations, which suggests they don't actually take that lack of validity seriously at all.
In 2017, the journal Autism Research published a letter to the editors, by Lynn Waterhouse, Eric London and Christopher Gillberg, entitled "The ASD diagnosis has blocked the discovery of valid biological variation in neurodevelopmental social impairment" [0] – the authors argue that focusing research on a diagnosis which lacks validity is a scientific dead-end, that it is no wonder that it has failed to produce the results which its promoters had promised, and that the field is going to continue to be fruitless until research is re-oriented away from its current focus on an invalid diagnostic category. And I'm sure they'd say that, while their letter was about ASD specifically, the same is likely true for many other DSM-5 diagnoses as well
> Most of them are just clusters of highly correlated symptoms without a clear, objective causal link to any underlying pathology.
How strong is the evidence that these symptoms actually are "highly correlated"? To take ASD as an example, the diagnostic criteria refer to problems in two separate domains – social communication, and restricted/repetitive behaviours/interests (RRBIs) – with problems in both domains required for a positive diagnosis. This linkage is justified on the grounds that the two symptom domains are "highly correlated". However, the scientific evidence for the strength of that claimed correlation is actually rather weak and lacking – see [1]. And if that is true for ASD, it is probably true for many other diagnoses as well
> This helps to facilitate communications between providers
If the point was really to facilitate communication between providers, a better approach would be to just provide a list of symptoms for each patient/client. DSM-5 diagnoses are less useful than that, because many of them are defined in terms such as "Exhibits at least M of the following N:", so two people can both have the same diagnosis without actually having any symptoms in common.
> as well as in generating the documentation required by payers to justify insurance claims.
Well, here we get to the real point of diagnosis – a collection of cultural constructs in which policy-makers have faith, so they'll construct policies (insurance, disability service funding, research funding, drug approval, etc) oriented around those cultural constructs – and they display a studied ignorance of the lack of scientific evidence to support what they are doing, and also the very real possibility that doing things that way is at best useless and at worst positively harmful (see [2])
