Magic also helps. Remember that magic takes place before movement, so a sudden barrage of Seeking Arrows or Earth Attacks might halt or bisect an enemy army when the surviving leaders blissfully continue their move orders while now-leaderless troops remain.
__________________
Are we insane yet? Are we insane yet? Aiiieeeeee...
About PD usefulness (except to tick AI ), it mainly depends on *what* you get as PD : Ulm PD is strong for example, and Vanheim or Marignon ones are good also. But Arco, Pythium or Machaka get crap that won't stand against anything..