At lower levels, or for characters with lower CON or FT, casting harm can drain their FT very quickly. Also before you can get your MA up you'll meet foes you want to disintegrate (ore golems). I'd suggest mages to learn bolt or fire flash for early game mass destruction. Note: mass destruction spells won't harm you or your party members. Only your enemies, objects and innocents.
I agree fireflash is great for groupes of monsters and you get the hit exp bonus as well as the kill which lets you level up quick. plus harm can be shrugged off by monsters with a high magic resitance ( i am thinking here about the dread crystal spider in the wheel clan mine). I never got into the lightening bolt spell. it is not really cost effective. if I want salvage harm at 5 ft v. bolt at 25 ft is the better deal.