None. When people say "life would be more boring if we all thought the same", that is really the only con. The whole point of debates should be to get the right answer. This movement of debate for the sake of debate as if politics or argument were team sport rather than trying to make the smartest decision is a very dangerous trend. We should derive entertainment from other things, arguments are supposed to have a purpose, and if no one is willing to change their mind such purpose is not being obtained.