Is Multiple Personality a Disorder?
Multiple personality, as viewed by psychology, is one end of a spectrum of dissociation. Dissociation is normal. And if one reads the section on everyday multiplicity, even multiple personality is relatively normal.
So why is it that multiple personality is now called dissociative identity disorder?
Think about families. A family is a group dynamic. In groups, sometimes small problems become magnified -- the interactions between people who have known each other for a long time sometimes develop patterns and habits that are difficult to break, and sometimes less-than-functional.
Multiples are an internalized family (see also As Inside So Outside) and as such a person with multiple personality can suffer internally from all sorts of issues that many families face, along with some additional complications that come from sharing a body and having to divvy up time between several people to actually interact with the world.
Just as some families are more functional than others, some multiples are more functional than others. We don't refer to all multiples as automatically dysfunctional or disordered. However, like many family systems, even the functional ones, a multiple system could probably benefit from some good advice and help from time-to-time, hence a resource such as this one, where multiples who are healthy and who are not can get some advice on how to better deal with times of trouble and with the inevitable rough spots that happen from time to time.
It makes more sense to treat multiple personality as one of many different expressions of the "Self" in this world, and to add to the diversity of the human experience than it does to lump everyone who is multiple under a broad sweeping heading of dysfunctional.