Well we are a magical force so you dont have to kill the other guy/girl if he/she could do good things for you.... right ???
So why not a whole bunch of spells that do that. like charm or mindcontrol. If the soldiers/heroes are under control of a spell you should also be able to undo the spell.. So you get a whole lot of take over games.
About converting.. If I would be a evil wizard I could also opt for coverting those poor souls into a undead/monster army and send them back to my enemies....
because these drain on your magic/spells just like normal armies drain on your gildars you get more ways to play...
Bring on the hordes of undead/enslaved !!!!!!