That's not the issue. I don't know about you, but had I watched those ViDocs (which I avoided like the plague) I would have been waiting for each moment to happen in the game. From there, I would've been piecing the story together based on what they spoiled in those ViDocs. I never understood why they showed us so much story info when the people who buy Halo for the Campaign would have bought it regardless.
I hate when they do this in the movie industry too. For movies that I really want to see, I try to stay as dark as possible. You guys remember Cloverfield? Before that movie came out, JJ Abrams barely gave anyone any information about the monster or story, further intriguing fans. As a result, I hold that movie in high regards because of how it was handled prerelease, and it actually turned out to be a pretty good monster movie.