I don't know, obviously Batman is an important member, but the League was formed and has survived without him. Hell, the League almost disbanded thanks to his plans for taking them all down. MM lives for the JLA because that's all he's got. He doesn't favor Gotham or any other city for that matter, he is constantly working for the League, not dividing his time between his other dealings. He's everything the JLA is supoosed to be.