Film Theory: Was The Dark Knight's Joker Actually A SPY?!
1. The Joker Actually Wants To Save Gotham
As we've discussed in another theory already, the Joker actually works for Gotham positively. He cleans up the police force and local government of corruption, he brings down organised crime in a much more meaningful way than Harvey Dent (and is much more successful than Batman) and he teaches Gotham a lesson about morality.
He also gives Gotham's residents what they need more than anything: a martyr and a monster to loom over them as a deterrent. And it works for eight years (until Bane arrives to enact Talia Al-Ghul's revenge on Batman). It doesn't fail because of a problem with the Joker's plan, it fails because of Batman's previous sins.
The image of The Joker as a former Gotham resident who returns to use his certain set of skills to save his home before disappearing once more is certainly a compelling one. And it works a lot more than the pretence that he has no plan at all. He's no agent of chaos, but he could certainly be an agent of some description.