That would work if people could disregard their feelings and considered only the logic of arguments. But that's not always possible, as much as we try feelings can't always be controlled (though learning to control them is definitely a worthy pursuit). By trying to be kind we're minimising the potential for feelings to get in the way of the logic. Sure it adds overhead but the advantage of this is that opinions and view that might otherwise not get expressed do.

Also how many of those pricks that are often right actually are often right and how many are just so much of a prick no one wants to challenge them?