I am not a Multiple, but I was married to one for 15 years until she died recently. I have a degree in Physics and Math from the University of California and have taught in junior high school. I spent almost every day with my wife for 15 years. Being Multiple is a very real condition. I am absolutely certain of that and will debate it with anyone. Multiplicity is as real as anything I know. I was trained as a scientist. I observed my wife for 15 years. She and all of her "insiders" as we called them, were very real.
But it is NOT a disorder. I believe this very strongly. My wife had 72 individual personalities. She had emotional problems, but they were not because she was a Multiple. Her problems were caused by her experiences as a child. I worked with each individual in her system and she was able to overcome many of her problems.
If a person who is not multiple were experiencing emotional distress because of something in their past, a therapist would work with that individual in overcoming that distress. To say that a Multiple must integrate and that will solve the problems is totally wrong. I had a wonderful and rich life with my wife and all of her "insiders". The only problem that would ever come up would be if there is not communication among everyone. My wife had very good communication and was very co-conscious. I worked on that with her very much. We had absolutely NO problems due to her being a Multiple.
Therapy for a Multiple should focus on communication first and then on the emotional problems of the individuals. Once you get past the idea that it is possible for different minds to be in one body, the therapy would be pretty much the same for multiples and non-multiples. However, it is important to know how each persons system works in order to be effective.
I would encourage everyone to learn as much as possible about Multiplicity/Plurality. There are many websites available. This is a good start. http://healthymultiplicity.com/
The human brain is an amazing organ. There is so much we do not know about it. I find it hard to understand how some people can say that they are certain that Multiplicity does not exist. I would ask them how they know that with absolute certainty. I would also ask them to watch Chapter 11 of the "Ascent of Man" called "Knowledge or Certainty" with Dr. Jacob Bronowski. http://www.youtube.com/watch?v=hqOfqBoafTc
Much more research needs to be done, but the most important thing is to get Psychiatrists, Psychologists, Therapists, and the general public aware and familiar with Multiplicity. It is strange only because it is unfamiliar. For me it is is completely natural. It is an amazing aspect of our amazing brain.