Some forums do not even allow modification of previous posts. I suppose any deletion or modification runs the risk of producing non sequiturs. Also, if we do not indicate to whom we address a remark, a deletion can make it seem as if we are referring to somebody else. I have seen some hilarious results of this on other forums. I think we all have to use common sense and, if we wish to materially alter the sense of something we have said, then it is better to write a new post to that effect.