Insomnia doesn't seem to come up much in discussions of Dark Knight. But the action in that movie is supposed to be driven by fear that discrediting Al Pacino's cop will lead to the release of criminals. So Al Pacino gives up his life to preserve justice/his image simultaneously, while Hilary Swank looks on in shock. In Dark Knight, the success of the Rico prosecution hinges on Dent's image, motivating Batman's "sacrifice," while Gordon looks on in grief. I liked Insomnia better. The only ambiguity was about Pacino's motives.
If you somehow believe that society is an illusion held together by mutally agreed upon symbolism, I suppose the hysterics about Dent's image wouldn't seem so absurd. I don't understand how anyone can be so confused about the world, though. It's like thinking that the Joker's nihilism is a real philosophy or a real mental illness or a real anything.