True, but at least in the case of iMessage, that would also mean not being able to text all while not within Wi-Fi coverage (since Apple absolutely refuses to let people actually send an SMS to a number it considers to be participating in iMessage).
(IIRC, my plan with Cingular didn't have any texts built-in, so they were 10¢ a pop, in each direction. I could send many thousands within the data plan's couple of gigabytes.)
Are you sure? I remember iMessage falling back to SMS if for some reason it can’t get through on iMessage.