I remember enemy idle dialogue about how badly injured those who survived an encounter with the Bat were. Apparently that only happens to no-name characters though.
Because he's a pompous twat that needs to go around beating up poor people to get his kicks. If he crippled his enemies he'd look like even more of an arsehole when he went back to beat them up in their wheelchair.
This is ultimately what I see at the heart of Bruce Wayne. I love Batman as a character but when you see the sometimes needless brutality you can not help but think he is a Scarlet Pimpernel working for the aristocracy, keeping the order he craves, and simultaneously working out his anger on people who could never have the chance to become the perfect crimefighter.
But, from a sheer numbers perspective, he should have ghosted Joker long ago. I understand why, as has been already posted, but even Batman would see the numbers of thousands of people killed by the Joker and probably kill him.