But isn't it the scope of the game to kill the enemies' mages? Hence the name "Hero Mages"?
Well it may be the goal in Mage Duel or Assassination, but not in the kill everybody typ. And even if Mages could be revived they would still remain the main target, but comebacks would be a little easier and disregarding lonely guards a little more dangerous. For the side with mage running and collecting Spells could become costly if the other guy can revive his mage and for the side without a mage the nature of Spells he is dealt still retain some sort of significance.