Over 80% of people who meet criteria for one dx meet criteria for more than one.

There is much debate (and there has been much debate for a while) about the utility of diagnostic categories compared to particular symptoms that a patient is experiencing. Diagnostic categories have a significant overlap (such that comorbidity is often a matter of counting the same symptoms two or three times) and different clinicians tend to see the disorders that interest them (inter-rater reliability on psychiatric diagnosis is poor - not much better than chance).

This is complicated by the issue of health insurance and clinicians have considerable scope to alter diagnosis to cater for the patients interests with respect to what the insurance will and will not cover.

Treatments tend to be catered to symptoms rather than diagnostic categories (e.g., medications and even forms of therapy).

Hopefully diagnostic categories will be done away with in the near future. What is holding up progress is controversy over how we are best to capture the symptoms.