When you cast a touch spell, you are considered armed for the purpose of the touch attack, so you don't suffer an AoO.
I, personally, don't like it that I almost have to have an evil cleric around in order to make the climatic battle, well, climatic. I like having a dragon, arcane caster, heck, even an evil warlord as the Big Bad. Since none of them can cast Heal, I don't like what happens when they are hit with a Harm spell. And despite what you think about ME vs THEM, my players actually appreciate tough, climatic battles. They tend to get bored when things are too easy - to the point of neutering themselves by not using certain items, etc when it makes things too easy.
IceBear