I don't know if I would say "necessary" but some people do have trouble with their bands coming off. I wear mine on the tighter side, so I ahven't really had that, but if you bump it on somethign, it could open. If you want the peace of mind there is nothing wrong with them.
Disney used to sell sliders and such, but they haven't in a long time. My 50 Anniversary band did come with a metal one.