> However, you can compute exact letter probabilities for his rack and strategize accordingly.

I think the best programs run Monte Carlo simulations on the contents of their opponent’s rack. That includes modeling their opponent’s moves, and thus improves on just computing these probabilities.

As a simple example, if they played a move, but, if they had an ‘e’, could have made a much better move, chances are they didn’t have an ‘e’ when they made their last move, even if that, a priori, was still highly likely.