Am I right in thinking that Bale is the only actor to play Batman in the movies (not counting the 1966 one) after an audition? I think Burton had Keaton in mind, while Shumacher seems to have offered the role to Kilmer and Clooney respectively after their predecessors quit or were fired. Affleck, as you say, was offered it, but Bale auditioned, along with a lot of young actors around that time.
But you still do auditions in case it doesn't fit. Well, usually you do, to save a lot of money.