Holy Avenger is approximately as useful as Fire Shield, which F2 mages love to cast when unscripted. (It costs fatigue and a turn of mage time, and maybe will do a few points of damage to the first unit which waltzes up and kills the mage.) The usefulness of offensively-defensive buffs is very situational, but I guess mages are just paranoid about their own skins and don't realize that Fire Shield doesn't also give an Awe +0 effect.
The only solution I know of is to either script or have enough mages that you don't have to care about inefficiency in casting.
-Max