30 years? Nah man, MPD is something made up by Bendis for the Ultimate before he transferred to the main line in his series making Marc delusional about talking to Spider-Man, Wolverine, and Captain America.
He might be less disturbed in later series, but I haven't checked those entirely, it seems that most writers starting with Brian Michael Bendis don't really know how to write him. I reread the series I love about him and what I saw is him using these identities like any superhero switches between masked identity and civilian one, or like the Chameleon pretending to be different characters without losing sense of his true self or what the purpose is of these alternate identities.