In an attempt to help you the reader see this issue my way, I first want to paint you a picture. Suppose you were the heir to a large sum of money and an empire at a young age. Suppose that your good looks and charisma had given you celebrity status - along with its fame and the ability to be easily recognized in a crowd. Try to think of people who might fit this description - perhaps Paris Hilton or Ivanka Trump.
Now suppose that for whatever reason, you wanted to go about secretly doing good things and stopping bad things from happening. You are so driven by your cause that you even break the law from time to time. You are, in fact, a vigilante, so naturally you don't want anyone to recognize you. A disguise would only make sense. Picture that time that Tyra put on a fat suit and was appalled at how mean people were to her. (No, I don't watch Tyra, but I happened to see part of that one).
Ok, so there you are, a celebrity in disguise and a vigilante out to change the world, and it comes time for you to interact with someone. How are you going to talk? In your normal voice? The one that people know really well from all those times you've been on TV? Really? Good luck with that. You'll be outed in your first month.
Or, would you perhaps try to disguise your voice? If so, why not try to use a voice that somehow compliments what you are wearing?
Mad props to Christopher Nolan for making Batman yell every time he speaks, and mad props to Christian Bale for doing it. The Bat-Voice was exactly the way it should have been. Honestly, If I were suddenly grabbed by a great big bat-like person and instantly zipped to the top of a tall building, and then that man was growling at me - I wouldn't stop and think "Hey, that's just cheesy..." No, I'd probably pee my pants and so would you. At a time when adrenaline is pumping through your body and all your senses are heightened, the experience would be burned into your memory. Had Bruce Wayne used his normal voice, the next time you heard him on TV you would instantly know it.
One thing I learned in film school (which is what I call the film class I took in regular school) is that the true way to measure the quality of a movie is in its believability. The Bat-Voice makes the Bruce Wayne/Batman character seem more real and therefore more believable, whereas a regular voice would only make the scenario less plausible.
By the way, Christian Bale lost his voice three times during the filming of Batman Begins. That's dedication.
