Plymorph not working

One of the cases would be in the CreatureEventAI - the polymorphed target is trying to cast a spell. I.e., CreatureEventAI::ProcessAction:

...
case ACTION_T_CAST:                     //11
...
CanCastResult castResult = DoCastSpellIfCan(target, action.cast.spellId, action.cast.castFlags);

...
case CAST_FAIL_STATE:
    cmFlags |= COMBAT_MOVEMENT_SILENCE;
    break;
...
if (cmFlags)
{
    if (!(action.cast.castFlags & CAST_NO_MELEE_IF_OOM))
    {
        AddCombatMovementFlags(cmFlags);
        SetCombatMovement(true, true);
    }
}

So the spell fails with CAST_FAIL_STATE, resulting in non-zero cmFlags starting the combat movement.

Is this code really necessary? E.g., in mangos-zero there only a DoCastSpellIfCan() call without hadling its results.