~snipe~
I suppose we all know now, even you, that it was an April fool's joke pulled on the forum by Theymos and most youngsters were fulled.
Now, for what is what, the IQ test could be questioned for certain flaws and that counts towards reasons why, it won't have been the best practice to this measure and I would give you reasons to its flaws.
1. Every test system has a time frame and the IQ test system came with none execept, a user posts in an intended thread before your submission was to come in.
2. The questions could be rolled over for a new one should you find it difficult to attempt, just a simple refresh or back and forth and it's changed.
3. Users have got Google to aid them all the way. Simply copy and search the question on Google search engine and suggestions would come pouring down in minutes.
If a user is so much concerned about knowledge of the forum precisely, aside from getting them off personal interaction with the forum itself,
TheBitcointalkShow would be a good place to look.