I think it is fairly common to be diagnosed solely on clinical presentation. The T that first picked it up in us did that. When we moved cities and got another T we told her we had DID, and she got us to do the DDIS to confirm it... but it was mainly out of interest, I think! (It was interesting for both her and me).
In this current return to therapy our therapy can be fully funded if we have a current diagnosis that is directly caused by the abuse. Given that the previous diagnosis was over 20 years ago it needs to be reconfirmed before they will fund any more therapy. Thus the testing.
I think generally speaking it isn't necessary or even really useful to confirm DID through specific testing.
Gawd, wouldn't it be 'funny' if after all this time it was found we don't have DID at all? That would sure put life in a spin!