[thejeff]

I didn't know that PD helped lower unrest.

More generally, 1 PD is essential, anything beyond that is situational. Don't rely on it to stop any significant enemy attack, but it's worth it if it can hold off most random events and the lesser remote attack spells and small raiding parties. Especially early in the game. Even if all it does is make your enemy send less, but larger, raiding forces. That's more cost for less provinces.

Later on, when raiding parties tend to be thugs who can take any reasonable PD, it does become almost useless.