I get the idea that it makes it more personal, because then Joker knows where to strike, emotionally speaking.

But I don't like it. I think their conflict needs to remain between the Joker and Batman, with the Joker having no interest in the actual person beneath the mask.

Plus, of course, as with every villain, there's the long-term effects of the identity being known. Joker's got his own weird sort of way, so maybe he for some reason would never ever reveal it, even after targeted jabs don't work. It's a hard sell, but maybe. But this happens with other villains, too, sometimes, when it just doesn't make sense. So, I really think villains should only very, very rarely know heroes identities.