He won't use it because he probably uses skull of young akamira. In some peoples opinion, that hamlet is better if you spec a certain way, because you end up with a more consistent formula for skill regeneration. With that helm, you can spec so that your melee regenerates your grenade and your super, your grenade regenerates your melee (using certain legendary arm perks) and your super regenerates your grenade. Basically every time you use a skill, you are reducing the cool down on something else. Obsidian mind can't do that was well as skull can...