Originally Posted by The Eradicator
You're right to a certain extent, but just imagine Bale's Bruce and Heath's Joker sharing a prison cell. That would have given TDKR an entirely different kind of psychological depth. It wouldn't just be about the villain helping out the hero but rather the hero learning something existential about himself from his arch-nemesis on a very personal level.
Think of the all the possible scenarios that could have played out from this.
That would have been pretty fascinating, but why the heck would Joker help out Batman and help his new playmate escape?