Hardness comes into play in more than just chopping up your yet-to-be-gained treasure. You can cut through doors or other barriers, disable vehicles or siege engines, cut some ledges into a rock wall for easier climbing, deal more damage to animated objects, sunder a giant's weapon...
Given the price of adamantine goods, few NPCs will have them (at least until uber-high levels where it doesn't matter, since most PCs will also have them, and the high Hardness of their adamantine items will nullify this ability). Since the PCs get to use their items in every game session--and NPCs tend to get killed, eliminated, or just left behind each session--the PCs get more utility out of this new ability.