I’ve run into this problem a couple of times. Often people write posts that contain many parts: replies to multiple other posts, an answer to one question and a question of their own or basically anything that could just as well be split into two or three separate posts. What do you do when one part of the message is on topic and another off topic? Or when you would like to branch out a discussion that has been developing inside another topic, but most of the posts contain stuff for the original topic as well?
I suggest a feature to move only part of a message, in effect splitting up posts.