Passed balls with runners on base equal a stolen base. Who allows more bases? A great catcher works with her pitcher AND the umpire. She understands the game and can identify a batter's weakness by watching her swing and location in the box. Stolen base attempts don't happen that much with a catcher who has just an average arm. I'll take the average arm, but knowledgeable catcher over the one who can just throw out runners. But there is way more to it, than just that.