You guys are correct. Spielberg does talk about his films in documentary so it's not like his thoughts go untapped. And the docs are usually great. But the difference for me is this: A doc. when it's thougtfully done and not just fluff is great. But, docs seem to be fairly organized and scripted, wheras with a comentary the participants have a 90+ minute canvas to speak about what they want (I am aware that participants are often given notes for sessions but for the sake of arguement......). I feel with a commentary, you stand a better chance of getting more subtle details about the film that wouldn't necessairly be bothered with in a doc. I definately noticed some of this in the Alien Quadrilogy set. For example, the docs in that set hardly ever talked about the film processes used in shooting, but those detials were mentioned in commentaries. I know that there were story elements as well that were mentioned in commentaries that were not as apparent in the docs (maybe it was intentionally edited that way, but I don't know).
Also, a really great uninterrupted doc. is a lot more rare in DVD than a good commentary. I do like both formats alot and we are lucky that the Spielberg docs are really good. But I can see why people would be excited should he ever decide to do commentary on his films.