Like D2786.
If your units are in combat and any siege unit appears in your vision range, then all of your units will attack siege units avoiding all other units, losing a lot of effectiveness (units Will try capture no kill) and can be killed on the way.
These units simply have "<PreferredClasses>" for being supposedly good against siege units.
Following this logic, Spear cav should have <PreferredClasses>Cavalry</PreferredClasses>, spearman should have "cavalry", sword cavalry should have "archer".
Well, that doesn't seem to make sense to me. Let the player make his choices.