That is Humpty Dumpty reasoning. Studying MPD does give us information about how personalities are formed and what the necessary and sufficient conditions are for being a person. Studying MPD also raises the important question of what we take a self to be anyway without being circular or assuming what we want to prove. Braude sees this question, even if I don t think he answers it satisfactorily. Finally, Braude also raises the important point that we should not use MPD to argue that we are really disguised multiples.
More work needs to be done there.
The real question is how we decide that issue without begging the question.