That is assuming she is really dead. The Joker is a master at manipulating people and scenarios to acheive his desires in unexpected ways. Wouldn't it be truly funny if he never killed Rachel, but only made it appear to be so, and drove Harvey mad with something that wasn't true? Doesn't that sound like the criminal psychotic genius we expect from someone like the Joker? I mean a common street thug could have set the expected scenario up, but a true genius would go beyond the normal setup, use her faked death to drive Harvey mad and torment Batman, and then once he was in the depths of his madness reveal her to be still alive and torment him with the fact that he committed his crimes of insanity for nothing, for a lie. And what would that do to her? Once she knows that the man she loved went crazy and killed people in her name, and then Batman "killed?" him? Wouldn't that maybe drive her mad and cause her to take revenge on Batman?
I mean all we saw was a building that the Joker gave us the address for blow up. No body, no evidence.
A little off topic, but hey, what about that Russian ballerina? She was brought up too many times for it to be something of unimportance. Sometimes the stuff is right there, right in front of your face, and you never notice it. Just like the Joker, Nolan is a genuis too.