Keaton can still be effective Bruce Wayne and Batman even if the movie was a silent film. Zero dialog and all.
Bale? He too can do it, but he can't project the Bruce Wayne/Batman internal psychological battle properly.
Bale growls and lisps. Keaton didn't have to do any of that to get his points across. Keaton just makes a slight change of voice when he's in the suit to something more raspy, confident and concise. A quick flash of his crazy eyes, standing there in the alley, and you know you don't wanna mess with him.
With Bale, I almost feel like I just wanna poke him a bit and see what makes him tic, because his entire outwardly "brooding" approach to Batman is self-defeating: he will never kill you, so make fun of him all you want.