Oh, the centurions certainly can take indep provinces alone. The holding is for both letting the enemy troops spread a little (you know, heavy infantry is a bit slower than light, and cavalry is faster than anything) and making it harder for the opposing archers to hit. The point would be that the centurion/s routs the enemies one squad at a time, without facing them all at a time. (Turin might have another reason for it, though, but that's why I do it.)
Works on most independent settings, at least to an extent, although as the strength of the indeps goes up the chances of a centurion dying go up as well. You can add to the force as many centurions as needed, though, since they don't rout even if some die; you could also equip some of them: only some, so that if that particular centurion dies the others can pick up the stuff.
Agreed that anyone would have a hard time beating any undead theme played by an above-average skilled player in a 1v1. I still would rather face SG than AE Ermor or CW Pangaea
