Well... :
uts on nerd hat:: the term "dissociative identity disorder" (or multiple personality disorder, whatever) was coined around the time that the DSM (diagnostic and statistical manual of the american psychiatric association) came out (sometime in the early 70s??) because before then there was no standard codified way to diagnose someone with mental illness. People who now would be diagnosed w/ DID previously would have had diagnoses running the gamut between bipolar disorder all the way to schizophrenia (and those two got confused a lot as well). I think the reason DID is a more appropriate term is that there is one central, true personality and when someone "splits" they split off from the true one. It's not like someone just randomly cycles around. It's such interesting stuff, but also sadly misunderstood.
Oh and yeah, I think it was a good movie. Some of it was scary but most of it just interesting stuff. Good plot twists!