For me GDD are good in 2 cases: "big" teams (+3-5 ppl) or big games.
I'm a game designer myself, and when I was working in a studio we used game design documents to let the rest of the team know how the game work, because we had several developers from different countries, several artists, etc. Also when we made branded games, our clients liked to check the document to feel that we made good work
Now I am full time indie developer and I make little html5 mobile games for game portals, I never write GDD again. But of course it's useful to make little documents to take notes though.
Also if you plan to make a big game with scenario, huge gameplay mechanics etc, you'll have to write design documents.Lowren2013-05-09 22:40:46