Don't under estimate this skill, it makes for a great opener against mobs. My Blizzard can crit for up to 450 damage at rank 14. Also, this doesn't seem to effect Cure.
Profundity seems to only effect directly offensive skills. Shield Spikes, Cure, and Stoneskin are not affected. Also, Burn and it's kin seem to only gain a boost to the DoT damage, not the reduction of fire magic defense, but i'm not certain on this.