We all have a Big Harvey hidden inside that we repress to have normal lives so I think it's a far more interesting version than Harvey Dent becoming an angry, rage-a-holic with no history of an evil, dual personality.
The seperate personalities represent everyone's capacity to make good and evil decisions. Without two personalities it's just a pissed off guy with a coin.
We see Harvey turn to evil but it isn't clear it's because he had evil inside of him all along waiting to get out (like all of us including Batman). He just turned evil because he's angry.
You think everyone has a split personality disorder except for the film version of Harvey Dent?
...what?