[st.patrik]

Certain nations I always buy defense for, in almost every province. Ulm, for example, gets those arbalests, which is a great backup force to a standing army, and is pretty good on its own, since sometimes you can kill enough the first volley to rout the attacking force (I'm thinking CoW/W here).

Other nations I pretty much don't spend much on PD unless I am expecting a big battle and want some fodder - in which case I'll buy some PD and then put my real army on the flanks. Hopefully the PD will soak up most of the damage (which is all they're good for) and my main force will survive mostly intact.

The other use for PD is to buy a couple of points just so that if you get attacked you'll know exactly what hit you. So I quite often do this, since a point or two costs hardly anything.

That's in MP of course - in SP the AI is too stupid to realise that your 21 PD is a pushover, since it's composed of militia, so it ends up being an effective deterent (for a while).