I'm surprised that no one has mentioned what I take to be the best reason to build Sun Tzu's: To keep your rivals from building it. You see, Player-run states have the sense to build barracks early in cities that produce military units. The AI is not that clever, and it builds many regular units even late in the game. This makes them very easy to defeat. Sun Tzu's has the effect of covering up for this strategic mistake on their part, and it makes defeating them much more difficult.