[Endoperez]

As for buying PD:

level 1 always shows you who conquered the province. Spending 1 gp for that is very much worth it.
1-3 PD, depending on nation, is enough to beat enemy scouts, small groups of fliers, etc solo guys that your enemy might want to use. In multiplayer, most provinces have PD in this area.
11 PD has a small chance of finding and fighting against stealthy enemy forces or commanders. In single-player, buying some PD in provinces neighbouring enemy nations can keep the AI from attacking you.
21 PD has a bit better chance at finding the enemies, and it starts getting troops of better quality. All nations get another type of commander, who is sometimes a mage or a priest. This much PD is rarely used even in SP, but in some cases having lots of automatically replenishing units is worth it even if you can't script them - even in MP.

Quality of provincial defense varies wildly between nation. The nation list at the end of the manual also describes the units the nation gets from its PD. Some nations have excellent PD, some have useful PD, some have very, very weak PD.