Good question. The upper, springy ones I can understand. There's always the possibility that one will break, or that the the silver contact will eventually get pitted enough that it won't polish up nicely anymore. None of this has ever happened to me, but I do have a few spare upper ones myself, just in case.
However the lower one? It isn't flexible, and it doesn't make/break a circuit, so I can't really imagine the circumstances under which it would fail.
Yeah, the ones I use the most all have some character marks; even the SS...