The definition of "harmful spell" is rather broad, and includes passive spell damage from buffs such as Thorns and Molten Armor, as well as non-combat actions related to quests (like picking up chickens for Chicken Party!).
Effective Spell Power
The effective spell power granted by this item is affected primarily by cast time. This trinket benefits more from shorter cast times.
The formula for calculating the amount of effective spell power granted by this trinket is:
E = 590 * 10 / (45 + (t / 0.25))
Where
E = effective +spell power
t = cast time
The following Java Program can be used to obtain a table of effective +spell power for different cast times.