I could see it being viable if handled tactfully. I wouldn't mind being a "guest" who in turn, could post in a guest general discussion section. Then once I bought my game - get key - come back to the official Skyrim or New Vegas thread and be able to post in the "member" section.
I see how this could alienate potential customers, but I don't need to post to review some aspects about a game I'm interested in. Grab any general discussion thread and most of the game's context/gameplay/story can be scratched upon in the first two pages of threads.
Then again, it might just come down to rights. I feel I should have the freedom to post here about whatever (within the forum rules of course) and not require a game key. I was thinking about and looking into New Vegas for a couple months before I actually got it. If I had to use a key, I would just go over to a smaller forum community and talk about it there. I couldn't ask specific questions to the "member" community on things that might make or break my purchase overall.
Great idea to make this topic it's own thread Guybrush!