Its all a bit strange...hard to know what to say really. The only times I've heard mention of Multiple Personality Disorder is when I saw an interview with a man who had some ridiculous supposed alter personalities, on Oprah - he talked about becoming a toddler while driving a car and nearly crashing it. Then there are movies about people who suffer from it.
People also argue that amnesia is fairly uncommon, don't they? or maybe not amnesia as such but that phenomenon of waking up after a coma and not remembering your own identity at all. Its usually short term memory thats lost and not so often longer, older memories?.
It is uncommon but following coma or indeed ECT, it can happen. It often returns though. People with Catatonic schizophrenia often lose the entire period of catatonia as the memories simply have not formed. Another odd one is the dissociative fugue, where someone is functional and "wakes up" in a different town etc. This is very rare indeed.