If you are assuming the Staff of Herding can open Rainbow Portals, like the ones Rainbow Goblins leave, you would be mistaken.
Rainbow gobs open portals to Whimsydale. The Staff allows access to Whimsyshire. They are visually identical but have different drops. Whimsyshire, the Staff one, has exclusive cosmetics. Its drops are like any other place. Whimsydale, the goblin based portal, has the insane crafting mat drops.