Let’s put a smile on that face! That was easier than I expected. I’m sorry.  
No worries. You know, I just realized that Batman’s greatest villain in “The Dark Knight” wasn’t the Joker.  
It was Harvey Dent. If Harvey Dent came into power he’d eradicate all the crime in Gotham in no time at all.  
Thus effectively nullifying the need for Batman, and Batman wouldn’t have any of that.  
The Joker keeps Batman alive, and Gotham never gets rid of all its crime. Harvey stays alive because he’d be a martyr otherwise.  
Of course if Batman keeps Poison Ivy alive she can paint one of her giant venus fly traps to look like Piranha Pete from Mario Kart.  
Pirahna Pete is not the hero Gotham needs, but the plant it needs. Wait. How did it go, again?  
The Joker is the hero Gotham needs? Exactly; the Joker did nothing wrong.