Is a diagnosis helpful or not?
In the past doctors sometimes did not tell their patients if they had a serious illness. This would now be deemed to be unethical as it would be considered the right of the patient to know a diagnosis, agree to a treatment plan and there is a culture of the patient and doctor working together to achieve the best outcome.
This approach covers both physical and mental illness, but there is growing number of patients who find a diagnosis of a mental disorder unhelpful. There was a very interesting article published online https://www.madinamerica.com/2017/11/re-abused-re-traumatized-diagnosis/ where a young lady felt that the diagnosis of Borderline Personality Disorder (BPR) was unhelpful as she feels that she as an individual is not heard or listened to and the answer has to be in medication.
On the other hand many people welcome a diagnosis which can provide a framework for the individual to understand their symptoms and what is happening to them at a very distressing time. The option of medication can provide hope for relief of symptoms and this approach can be very helpful both to the doctor and the patient.
From the perspective of counselling, it is vital that the client feels that they are heard in the counselling room. The counselling session is designed to be a safe space for a client to be able to talk about their life experiences (good and bad), and by recounting these experiences and being heard it can help a client make sense of their situation whether or not there was a medical diagnosis in place.
A counsellor is also in a unique position to be able to spend the necessary time with the client, value the client as an individual and not see the client merely as an illness.
There is no universally correct answer to the question in the title, but if you, or someone you know, has found that a diagnosis has not been helpful, then maybe consider seeing a counsellor in addition to the other support that is in place via mental health services. You should be assured of a safe supportive environment and a place where you can be truly heard.