I'm boring with only one diagnosis - BP2. So maybe I'm not the best to comment, but I will anyway!!!!
1. I think labels can be good and bad: they can give an explaination for what is happening, help guide treatment and give an idea of prognosis but they can be applied wrongly, cause stigma, and be used as an excuse.
2. Exact diagnoses are difficult with something as complicated as the mind, with spectrums of disease and different problems presenting in similar ways. Also so much is based on the doctors opinion that there is variation between what different doctors say.
3. More exact diagnoses may become possible for problems with a genetic/biochemical basis like some people with BP, some forms of depression, schitzophrenia, ADHD; but I think it is likely to remain subjective and difficult to diagnose disorders that are related to our interactions with society like BPD, PTSD, some forms of depression etc
Just some ideas