I actually think the subtext within the movie is far more cynical on Harvey Dent then it gets credit for. There are several clues/moments that show Dent's crusade is not pure altruism and is motivated more than a little by narcissistic ambition. Even just his monologue on the Batman and Roman suspension of democracy infers that he sees himself more like Batman than as a counterpoint to him, and he is shown interrogating a man at gun point long before he goes completely insane.

The counterpoint to Batman and what is needed in the end is not in fact Harvey Dent the person, deeply flawed and broken...but instead the idea of what Harvey Dent represented. The incorruptible white knight.