Review embargoes are a relatively new phenomenon (at least in this number). The spoiler argument is a pretense. People know what to expect when reading one and most of the time it’s not even that spoiler-ish since they are often supposed to give you a better idea of what the game is about. Very few critics would spoil the story for their readers.
So what is the reason? My best guess: Allowing them to uphold the fake hype that their marketing department worked so hard to create. And that is straight up dishonest. Instead of allowing their customers to inform themselves about their product they’d rather have them buy it blindly. Totally not cool.
Hm... That seems pretty reasonable argument. I didn't think that one could be so opposed to this though. It's just one of the marketing strategy and just like Renegader said, I would imagine people who really rely on review to purchase them after the release date.
Another possible reasoning could be that they want to have an explosion after the release date. You're right about holding the hype up. I mean, that's what game marketing aim for. If the game reviews come out too early, the hype wouldn't go down just because it had a bad review. It goes down because people become more informed on what the game is about. Simply just that. That's why they always try to not tell everything about the game all the time. We didn't know about werewolves until recently even though people constantly asked them about werewolves. I guess now that it's so close, they couldn't hold onto it.
After the game release date, a lot of people will be playing and will be talking about it. If the game reviews come out during that particular time, people who weren't interested in Skyrim will fall into the massive talk. For now, it's pretty quiet. See, this forum isn't as active as it can be because Skyrim isn't out yet. If game reviews come out now, the readers (like people who don't know Skyrim or forgot about Skyrim) will read it and then go back to forgetting about it because the game isn't even out yet. Not a lot of people are talking about it so the chances are, Skyrim won't come across their mind. Even after the release since they'll think like 'Oh, it's skyrim. I read some reviews about it'. They already forgot what exactly the review said, and put it aside.
If the readers read the review after the game has been out, they'll fall into it almost immediately. They read the good reviews, go find the game, 'oh hey, the game is out so everyone is talking about it. Maybe I should browse its busy forum to see what it's about.' The readers will also see the comments on the game review, seeing a lot of people agreeing to such a positive review. It will go on until the person becomes sold and buys the game.
Well, that was a long explanation. It's not all that obvious and I did stretch out some assumptions but I think they are quite reasonable. One may view review embargo filled with skepticism at first glance but I think it's a very subtle marketing scheme. It might be annoying to wait for review for couple of days but of course, there will be more sale for Bethesda. Of course, they could use this to 'trick' people into buying their bad game but I really really doubt Skyrim got bad reviews and people will regret buying it. :]