Well, I gotta rent Metroland to hear Bale's real accent. He was also good in Laurel Canyon.
Christian doing interviews in the accent of his character is probably a way of protecting his private self.
Pee Wee Herman (Paul Reubens)would only do interviews if he could speak in Pee Wee's voice.
I think he does it, because its easier for people to grasp and understand, and they won't get confused.
Think about it, Batman with a welsh accent, even if its not in the movie, if he is promoting the film, it might turn people off.
He's a smart guy, he doesn't stay in character per say like Pee Wee Herman, but he wants to sound like him and keep it close to the character.
Thats a pretty darn good work ethic and consideration if you ask me, thats going above and beyond the call for a film.