Well, Pythium Prov Defense is pure crap...
You can use that to grab provinces with stealthy troops or very small armies.
I've seen a 4-men Ulm army rout a Pythium Prov Def of 21 !
As a competent Pythium player won't invest in big PDs it also eases your scouts/spies/assassins work
Note that Arco doesn't fare much better here. But hey that's a weakness !