Hmmm does Batman need the joker?
No, he does not. But he (Joker) clearly is there anyways because when it boils down to it, Joker defines Batman. Many a time it crossed my mind why Bats just doesn't go ahead and kill him? Just be done with it, the guy is insane and very aggressive. Why doesn't he just end him? How many lives will that act truly save? A lot and Bats knows that. Joker is the kind of individual that would not think twice about the darkest thoughts that would cross a man's mind, and act them out. Ending the Joker however would truly be Bats worst nightmare, his very personal nightmare. Bats would eat himself up inside, at first he would fool himself into falsely believing he did humanity a favour and that is what heroes do. Right? But slowly overtime his strong will and logic would begin to crumble because at the end of the day, he did the very thing Joker forceshim to do. And thus he corrupted him.