I think there is a clear difference in "generational" game design. The old guard (the Gygaxes and Staffords and so forth) never really figured the rulebook would matter that much, since the GM figures it all out anyways.
The young guard (the Monte Cooks and so forth) grew up learning from the rulebooks though and as a result, while they still acknowledge the GM as being the most important part, there was a much stronger assumption that the game should be played the way its written down.
While I dislike 3.x D&D, I think its pretty clear that the game was meant to be played the way its written in the books. When I read AD&D 1e, its pretty clear nobody ever used any of that crap