So why are all the previews including the most recent rounds of them, all pre-alpha builds?
I think that the reason behind only letting pre-alpha or buggy builds be shown is the same reasoning behind only showing the xbox 360 version. Todd has stated that the 360 version is shown because they want everyone to be surprised at how well the 360 can handle the graphics, and to be even more blown away by the inevitably superior PC version(since no screenshots have been shown so far).
In other words, managing expectations. Has there ever been a Bethesda title that wasn't buggy at launch? No(in all fairness this belongs to the majority of games). Therefore showing pre-alpha buggy builds at demo centers will serve to make the final product look that much better on launch day, even if it is buggy.
Another way to look at it: If the media were allowed to play current builds and discover bugs in those builds, that would look at lot worse than the media playing pre-alpha builds that were already KNOWN to have bugs. In the latter scenario, they can just attribute it to the fact that it is an old build and those bugs will be fixed at launch. In the first scenario, even though it is a more stable build, but god forbid an ugly bug rears its head, then it's harder to put credence and confidence in the product.
Overall smart choice by Bethesda.
OR, I could be totally off my gourd and it is for an entirely different reason..
disclaimer: I love this game, I would run a 10k race if it meant I could get this game a day earlier, don't yell at me plz