I agree that the main media today is the mail.
But take this situation : it's new years eve, you're with a lot of friends, but one of them is not here (and he has not yet an iphone / blackberry / droid) at another party location, sick in bed or I don't know. You take a group picture, and send them by MMS a happy new year with the picture of you and your friends. He will get it right after you send it, and voila.
I agree that when everybody will have a smartphone with a correct data plan, it will be time to take this f***** MMS protocol and put it down to the garbage disposal of computer hall of shame. But right now some people still need this crappy protocol.