Labels can be vital when dealing with certain diagnoses. A person who is bipolar, but who is mistakenly treated for depression can be thrown into mania if only treated with anti-depressants. So, I suppose while it may not matter so much whether a T uses a diagnosis or not since he/she won't be prescribing medications, clearly a diagnosis is very important when being treated medically for a mental illness.

Also, my T needs that bipolar diagnosis so he can monitor my symptoms. People with certain diagnoses display some pretty clear warning signs when we are headed into an episode. The sooner a T can help a client recognize that something is going awry, the sooner intervention can happen to hopefully lessen the impact of that episode. My T's willingness and ability to recognize those symptoms as indicators of a bipolar episode has saved my life many times over the years.

So, I think the whole diagnosis issue is really dependent on what kind of diagnosis you are working with. I would be personally very wary of a T who basically blew off my diagnosis as not important to his handling of my case.