I fully endorse a design doc for any game you develop. It does not have to be complicated. It should, at the very least, help with this: brain storm ideas (in developing a design doc, you will often find new ideas for things like gameplay, art ideas, technical details). It helps keep you focused and on a direct path (but remain somewhat fluid). It can be really short or extremely detailed (would be needed for a team).
My design doc for my game is all hand written, with sketches and stuff. It sure did help hone in on a clear path.