[thejeff]

It's not really a straight math comparison, though.

Regular troops die and will need to be replaced if you're being hit with even small attacks. They also cost upkeep.

PD can't be moved and if you're expanding will be left behind the front useless except for raids.

And it's not generally a choice between building up PD or buying a few of the local troops to boost that provinces defenses. (Though adding some local archers to PD is often a good plan if you PD doesn't come with ranged troops.) It's a choice between PD and troops that can actually be part of an army that can fight an enemies main force.

In MP, it seems that PD should be high enough that your enemy has to use an expensive force to raid. PD will be almost useless against any humans real assault, but if he has to consolidate his raiding forces he'll raid fewer provinces.