A good argument, but I've got one to counter it (just to add more fuel to the fire):
Break-in should always soften the harshness in a speaker. Think of the tweeter as being a new pair of shoes. Before they are worn, shoes can be stiff and uncomfortable. The leather or canvas softens only after extended use. So with a tweeter, wouldn't extended use also "loosen" the mechanical components that are designed to flex? If this is the case, then we have an explanation as to why we never hear of break-in making the treble harsher.
Keep in mind that I am a fence-sitter on this issue.
