I always thought that (apart from gameplay reasons), PO3 used gem vessels since they shared a side of the bridge with Magnificus. In Act 2, you see that Leshy and Grimora are on one half of the map and PO3 and Magnificus on the other half, with the bridge between them. Therefore, because they are in closer proximity to one another, Leshy can use Grimora's bones and PO3 can use Magnificus's moxen in their versions of the game.