From an article by Steve Tomkins posted by Henry:

<< Scrabble meanwhile is a matter of fierce international championships, though it hasn't attracted so much interest from computers. Top players agree that one of the keys is learning all those two-letter words that seem to exist solely for the sake of Scrabble - like "aa" (a kind of lava) and "xu" (a monetary unit in Vietnam).

...

Nonsense! This is a little like saying that "top chess players agree that one of the keys to winning chess is learning the rules of castling." It's true, but it's a drop in the bucket. Every Scrabble player in a Scrabble club has long since memorized the approx. 95 two-letter words that are fair play in Scrabble. The top-ranked Scrabble players I've met have actually memorized *all* the words that are legal in Scrabble. (They have no idea what most of them mean, but hey.) --Dan