Hah, that's true, actually! Never noticed that. Although I suppose it's something to do with the characters themeselves - Nicholson's Joker (and to some extent, Ledger's) was a flamboyant man (the float scene, anyone?) whilst Batman, no matter the incarnation, is a shadowy, drawn-in figure, thus it can make it quite limiting re matching/ outshining the villains.
"The world is a fine place, and worth fighting for'. I agree with the second part.'