Hmmmmm...
First - if province has high PD, AI may decide not to attack right now and concentrate army in one armored fist. You can see this army - so you can move your forces or attack with FfA. Second - large army attacking defended provinces become smaller and smaller
I use this tactic when playing against opponents with weak troops: their armies cannot enter in my territory deeper, than 3-4 provs. And forth - my favorite protective enchantment (Mechanical Militia) work only if province defended with PD