I think the system for categorizing mental illness is vague and inefficient and often more destructive than helpful. These diagnoses can also be self-fulfilling prophecies - tell someone they have a particular illness and they may well subconsciously start manifesting the relevant symptoms. For these reasons (and perhaps others) I think professionals get edgy around the issue of diagnosis and start worrying about 'labels' etc. I have one DBT workbook that is supposed to help people with borderline PD but be useful for many others too. The main symptom it deals with is 'overwhelming emotions'. In a very basic and common sense way I think the issue of regulating emotions is core to many mental illnesses and all the other claptrap is superfluous.