That's just one of the reasons why we as geeks would see it. The GA doesn't care who's name is on it, all they care about is if the movie looks good and if they want to spend money on it. And Spielberg isn't directing. Perhaps that would hold it a little more in its favor, but Favreau isn't that name yet. "Oh, he directed Iron Man. That was good, but *insert opinion*"
I don't think adding someone's name makes a difference much. More so if it's a director, and less if that famous director is producing. It comes down to whether it looks good and people are willing to see it. Those movies that happen to be a success with the famous director's names attached isn't an exception.