[PrinzMegaherz]

I think PD is also very important in Single Player game, as the AI often gets ridicolous amounts of units, far more than you could have by that time, and floods your provinces.
Your main army cant be everywhere, so you have to rely on pd to at least hurt the enemy so much he needs to reinforce before he advances.

My best result was with 41 pd Ulm against Ermor. Nearly 200 killed, battle won.

Another thing to consider is that you dont need to rebuild your pd as long as you win. This is a huge advantage to your main army, where every dead soldier needs to be replaced. Just make sure your pd wins.