Harm will guarantee a knock down to 1 HP, whereas CS will only perform a crit. attack. Yes Crits are amazingly good, but that's not going to kill the bigger badder enemies in the game. Harm lasts for two rounds and you can do Harm, then Crit Strike, then make your attack; then just go all out on the weakened enemy and it's dead. This is also why Time Stop + Harm is ridiculously effective. With Time Stop, all attacks are automatic hits no matter what, followed by Harm, a touch spell that relies on the Thac0 of the caster to meet the requirements of the spell, this means that you get a free knock down to 1 HP for the enemy, and can just chuck a stone at it to finish it off.
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he got chunked in the final fight so I never did get to see his little post ToB blurb :P (never beat ToB before amazingly)
edit: okay I figured it out but if anyone knows I'd still like to know for future reference