The point you make about predictability is a good one. It's good if the AI would make some unpredictable, non-optimal moves. However, the pillaging behaviour of the AI is pretty predictable and non-optimal. You know that the AI will pillage with that lone unit that is just outside your border and you know that you will kill that unit when it does. I don't say that the AI should never pillage. It should do it sometimes, just not all the time. Another thing is that the pillaging unit is almost always a single unit or a few units of the same type (mostly horse based fast units). These groups can be defeated with minimal losses. It would be nice if the AI sometimes used a mixed group of 3 different types of units so that the pillaging group can defend itself. Pillaging is not a bad strategy in itself, but always pillage with a single (type of) unit (which is close to the strategy of the AI) is a bad strategy. The strategy could be improved without becoming predictable.