1) Your opponent spend his magic phase healing back units decimated by crumble; or
2) Your opponent spend his magic phase buffing/hexing/nuking so that he wins combats
Healing used to work in a world without step-up and extra ranks of attacks. It allowed undead to weather the storm until either their support got there or they got that one lucky combat and autobroke the enemy. Now, it's a waste of dice compared to the difference you can make with even a basic buff spell, much less the really good ones.