Ah, Rachel, Rachel, Rachel.
Poor Rachel. Because unlike most of our other major characters, Rachel doesn't exist. Rachel's not real. That is to say, she's not a part of the DCU.
And the universe has a way of taking care of things that don't fit in continuity, doesn't it? "Rachel Dawes" ain't an MJ Watson. I mean, it's not like anyone would ever dare take MJ's relationship with Spider-Man out of continuity!
Poor Rachel also had the misfortune of being portrayed first by an actress who wasn't very good and then by an actress who isn't really as attractive as she needed to be for the role. Of course, MJ has the same problem; it was just the same actress each time.
Rachel also had the problem of choosing between a gorgeous famous wealthy crimefighter and a gorgeous famous non-wealthy crimefighter. Throw me in that briar patch, please.
Of course, in a film like the Dark Knight, Rachel's dilemma isn't just a matter of personal choice. Her choice between Bruce and Harvey symbolizes the city's choice between extraordinary justice and ordinary justice. Ordinary justice is what the city is striving for. Bruce is hoping throughout the film that extraordinary justice (Batman) is just a means of getting the system back on track, so that ordinary justice can take over.
But the temporary means to an end have a way of turning into permanent methods of operation. Indeed, that's the very thing that Lucius Fox fears from Batman's use of the bat-sensor-web. And Batman's hope is in vain; extraordinary justice doesn't, in this case, lead back to ordinary justice, it leads instead to extraordinary injustice (the Joker). Once that's in place, extraordinary justice isn't merely an option, it's a necessity for survival.
Like the city, Rachel is grateful for Bruce/Batman, but doesn't want to embrace him as her permanent way of life. She longs for the normality, the stability that Harvey Dent represents (as does the city). She wants Bruce to represent that, to give up being Batman, but she realizes that's simply not going to happen (or, at least, she's not willing to wait until it does). Girls swoon for the bad boy on the motorbike, but they want to tame him eventually. If they can't, they almost always wind up marrying the stable, less dramatic guy.
Of course, that's simply what Harvey Dent represents; it's not what he is. He's not stable because the regular system of justice isn't stable; its stability rests on on the overall stability of society. The regular system is dependent on rules and underlying assumptions about the wholesale goodness of people (or, more cynically and perhaps more accurately, most people's innate understanding that the stability of society is more important to their well-being in the long-run than any short-term gains they might make in undermining it or breaking its laws). When that apple-cart is upset by an outside force (the Joker) and the stability of society is threatened, ordinary morality can breakdown (just as it happens with Harvey). Indeed, it's why society permits us to kill during war.
In the end, Rachel makes the only sensible choice: she chooses ordinary over extraordinary. Unfortunately, when living in an extraordinary world, the sensible choice may not be the right one.
