The cost to use it is, on the face of it, at most a couple of mouse clicks. How could that be higher than the benefit of letting every reader know why the conversation ended? Perhaps I'm leaving out some hidden costs here, in which case, what do you think they are?

As for the cost to implement, I volunteer to code the feature myself, if I can get a commitment that it will be accepted (and if someone more qualified/familiar with the codebase doesn't volunteer).