I picked up the skull on Gravemind last night and I could not drop it. I could change weapons no problem, but the Chief would not drop it. It was like a 3rd weapon in my inventory. It was kind of a pain, especially since this level is hard enough to begin with. My rotation went Plasma Pistol -> Carbine -> Skull.
If i’m not mistaken, that’s how that skull always is. It was like that in the original H2 release as well.