Just thought I'd add a nickel here...2 cents is just not enough these days.

First...anyone who sees a T, has a diagnosis. They have to write something down

ummmm....if you are dissociating frequently then technically you have a dissociative disorder. But there are, as JM said, many types of said disorder.
I think what you need to clarify (and correct me if I'm wrong) has to do with some other terms. Dissociation exists on a spectrum and is not always a disorder. (just echoing JM's post here.)
Check links on detachment...depersonalization, dissociation and dissociative fugue. Dissociation can exist as a symptom in many disorders.
As far as the diagnosis being important? I think that depends on if it is important to you and why you want to know. As JM said...you may just want to know in order to be more informed and know what you are working on.
Most T's will say it is not important. But know that it is important to them whenever they have to fill out any paperwork on you. So...you may want to discuss what diagnostic codes are being used by your T.
I don't know how old you are, but sometimes a label will follow you through your life when it may have nothing to do with the treatment you are requesting. (I am not trying to make you or anyone paranoid about this...I personally have had some negative experiences connected with my labels) So...with that said...I try to minimize labeling and just do the work. But, when I can, I try to have some control over what is being documented.
I personally believe that a diagnosis is much more important to a T than any of them would like to admit. They don't want to be judgmental but it is part of their job. In a way, isn't that what we all seek? Their judgment on whether or not we fit in? Are we normal? Do other people dissociate the way I do? Is dissociation normal?
These are nagging questions for those of us that are dissociative.
When I was diagnosed...and they way they went about it, (which is now considered "old school".) I took tons of tests that ran the gamut of written to physically invasive. I can tell you diagnosis was pretty important to all the Drs. observing. And it was pretty scary for me. I had a lot of trouble coming to terms with it. And yet...my gut knew they were right.
However...I NEEDED to know...I needed to know why I was doing the things I was doing. I wanted a concrete answer. The diagnosis did help me to understand the basis of my problems and gave me a direction to work.
Having a diagnosis is not going to fix the problems. A diagnosis is irrelevant because it is open to interpretation and can change. You still have to do the work and that is what most T's will focus on.
Does that make sense?
If your T is talking about integration of the self and parts. Then you have a diagnosis. You just need to decide if you want to know it. You also may not agree with the diagnosis and that is something you may want to discuss with your T.
SD