Certain models in AMA sanctioned competitions are required to have a spinner or safety hub. The safety hubs are not required for regular flying.

As long as you use your prop washer, and the nut doesn't "bottom out" on the shaft before the prop is tight, there's no real reason to buy anything. My trainer has just the washer and nuts that came with the engine. My Uproar has a safety hub. I know you do not use a starter, but, it works fine if the rubber insert is turned around so the small hole goes over the prop nut.