Yeah, you and I know that. But at the end of The Dark Knight
, the only people who really knew Harvey was "a lie" were the Joker, Batman, Gordon, and Gordon's son (if he was even cognizant enough to know what the fuck was going on.)
What after-the-fact opinions Gotham has about Harvey Dent in The Dark Knight Rises
has yet to be revealed, but I'd say that despite the fact that Harvey became a villain, the whole point of Batman's sacrifice at the end of The Dark Knight
is that the image of Harvey as a symbol of hope for Gotham was preserved after all.