Editing is where movies are made or broken. Many a film has been saved and many a film has been ruined in the editing room.
Editing is kind of a solitary job.
It's a lot of power to give the director to edit his own stuff. It's also a time thing: you don't want to have to wait for the guy to finish shooting before he starts editing.
Movies get found in the editing room. The movie that you make is not always necessarily the movie that comes out of the editing room. The trick is to perfect the movie that you have and make it the best version of what you've shot, regardless of what the intent may have been.
When you make a 3-D movie you actually have to plan the way the visuals look because there's a parallax issue, and there's an issue of editing; you can't edit very quickly in 3-D because the eye won't adjust fast enough for it.
I think that anybody that wants to direct, particularly writers, should spend some time in an editing room, whether it's a film of theirs or someone else's, or shoot their own picture on video and cut it.