You just posted a message and want to add extra information to your message. Lots of people just hit the reply button or even worse quote themselves.
In php-fusion we have a post merger option, but I don't use it for two reasons. [olist=1]I don't find it nice. It seems to have a bug when quoting a post.[/olist]
To emphasize the edit option this little mod shows a warning only when your the last poster in a thread. In the quick reply panel a message "Want to add something to your last post? Preferably use edit" is shown containing an edit link on the image. When you are not the last poster, no message is shown.