I have created a few smaller games on my own in the past. My approach has always been to create a completed editor where it has all the functionality needed to save a level file and load it into the game.
This has always made most sense to me but I keep hearing from people that a game is never fully done in the editor. I have never worked in a game development team and so I don't have first hand experience, but not adding everything needed to make the game to the editor just seams wrong.
Am I missing something? Is there ever a reason not to add a tool to the editor?