I agree that Thorns makes more sense in the off-hand (or even on armor).
I was suggesting to move Thorns to the Helm under the assumption that they keep the current division of Weapon/Helm/Other.
If they could, they should probably divide “Other” into Armor (which would include Chest Armor and Pants) and Jewelry (Rings and Amulets) as well, but I doubt that this would happen.