Also, some of the T3/T4 offensive spells are not as good as the T1 ones. For example, I believe Demonshreak changes its multiplier based on the alignment of players, so you have a 1/3rd chance (slightly less, since players/mobs aren't equally aligned distributed) of doing major damage. Unfortunately, harm can achieve a multiplier of 2.60 (or 2.70), and that's a T1 spell. Mana blast, Electric blast are also spells that have potential, but are outclassed by lower tier spells.
Demonshreak probably has a higher base that gets multiplied than harm does. If harm does 50 points base and gets multiplied by 3 then it does 150 points damage, if Demonshreak has a 100 point base and gets multiplied by 2, it still does more damage. And yes, those are only numbers. I've not seen code, I don't know the exact numbers, but my guess would be that demonshreak is T/whatever due to the base compared to harm.
Kvetchy