I've seen the AI rally, but remember once they've routed morale is always fragile, even if they turn, so would likely buckle quickly if attacked (i've had this several times with xbowmen) and with the morale modifier of being cut down by heavy knights from behind I can't imagine they would if you pursue them closely. Also once everyone's routed (and you've had the win screen) their penalties from other units routing mean they'd have to pretty suicidal to change their minds.