Sebastien and Rose have already come across the scenario where a one RLR user has something to add to a post written by another and the sensible thing to do seems to be to edit the post.
This is fine. Editing posts is obviously the right thing to do in some situations.
The problem is that the act of editing a post is not an obvious one to anyone other than the editor. You could make a really significant contribution to a post about something very useful to the lab but this contribution might not be at all obvious to any other RLR user.