You can easily wipe out fleeing units with repeating attacks or with more than one cavalry unit. Although I, being a very bad guy, normally do this it results in rediculously high casualty rates. It is true that ancient and medieval armies suffered by far the most when they broke and fled but the near extinction which is usual in RTW and EB is a little exaggerated. Of course, when fighting against AS, the Romans or Sweboz I'm very happy about it.