Let's say there are 50 guys which had luck in participating in the beta(0,5% isn't much).
Now let's guess at least 5 of them participated just for fun in the survey.
Also maybe 5 other guys just don't know that there is a beta forum, they just took the code and play Dawnguard.
I also saw a code on ebay, sold by some guy with about 5000+ positive ratings(seems legit!)...so maybe another 5 just sold the key.
And how many might just play the game and think something like "hey lol got the game 4free and saved 20bucks"....let's say at least half of those 50 guys

=== 10guys MAYBE reporting bugs...and this is VERY optimistic, probably just 5 or even less!
That makes me think about two options:
1.Bethesda is freakin stingy OR
2.they are stingy PLUS beta was just to get even more publicity....