In Golden Sun when it can first be bought in Lalivero, it probably would not be okay to buy because you can get two stronger pieces of headwear in Lalivero: The Jeweled Crown Artifact that can be bought as an Artifact at the same vendor, and the Warrior's Helm Artifact that can be found in a chest which is the single best headwear for Isaac to equip in the game. And with other equipment artifacts found beforehand such as the Lucky Cap, there seems to be no room for the Knight's Helm to be truly useful if the game has been perfectly played up to this point.