Personally I've always liked the idea that Batman needs the Joker around as a constant reminder of what would happen if he loses control. As Batman told Jason, he doesn't kill not because it would be too hard but rather too easy. If he started wiping out his enemies Batman fears he wouldn't be able to stop, would lose what control/sanity he has and would become a much worse monster than the Joker. Seeing a loner psycho clown who believes in nothing and kills without pause reinforces Bruce's belief that he needs his Bat-family around him and cannot break his oath not to kill for fear of what he could become.