Newbie question about province defenses

[Taqwus]

As Ashen Empire or Soul Gate, having a single point is useful to ensure that you have a commander on the premises. Otherwise, auto-recruited troops attacked without a leader will rout or fall apart.
For/against others, it varies. Against Pan or Caelum or anybody else making free use of fliers, PD by itself is very vulnerable because the PD does not put guards on the commanders.
PD does not affect unrest, I think. At 10+ or so (11?) it has a chance of catching stealthy units (not a very good one, at least for spies), which can be useful.