If I have replied to a node, it would be nice to be notified if that node gets changed. The worst case may be that the OP has edited their node in a way that makes my reply look foolish or irrelevant, but the more common case is that they have merely improved their node with no mischievous intent.
This would manifest as an additional tick-box in my Message Settings: "/msg me when there's a change to a post that I've replied to"