"dissociative identity disorder/multiple personality disorder" has been all but dropped by mainstream psychology. It's considered to be an artifact explained as being induced by a number of factors including attention seeking, desire to please therapist, extreme hypochondria and mimicry.

Whether or not it has been dropped by mainstream (source?) - you paint dissociative identity disorder in such a poor light - choosing only to focus on characteristics that make it controversial. I think you'll find that in fact there is simply a lack of consensis amongst psychologists for its basis - which happens to be the case for many 'disorders' in this arena.

As with many mental illnesses, whether or not it's real depends on what your definition of 'real' is. It's clearly a common pattern of behavior that's secondary to other factors, but you could say the same thing about lots of mental illnesses.