I can see both sides of the argument. On one hand 20 PD in every province seems like a waste of money later on, when more powerful armies and thugs can take it out pretty easy, but in early to mid mid-game it does make an empire gank more or less impossible to pull off on a grand scale.
In a game I'm in now one player had mostly token PD everywhere (like 1PD, occasionally 10, but monkey PD is always token PD, right?
), so I was able to sneak a bunch of guys in there and take like 18 provinces in one turn, combined with some remote attack spells (but not Arouse Hunger, that blows against monkey PD). If he had had 20 PD everywhere (or even 10), I would've run the risk of having some of my guys discovered. They still would've won their individual battle, but then of course my opponent can start reacting instead of being taken completely by surprise, and losing 3/4 of his provinces in a single turn. And naturally the supplementary remote attack spells wouldn't have been possible.
That's one important value of higher PD early on (though i can't imagine putting 20 PD anywhere much ever). It's value fades as the game progresses, but then so do a lot of things.
Edit: also there was another major player in the game who had put in higher PD, which was the deciding factor in my decision of who to attack, so it has that value too.