I'm putting together an old style cup and spindle BB. The bearings are nice cartridge type, which is nice for keeping the bearings from falling all over when you service it. However, is it necessary to have a spindle shield? It never had one, but my other old bike did. Logic tells me, it is a benny to keep water and crud out of the shell.
--"Will you have some tea... at the theatre with me?"--
1994 Trek 1200, 1984 Raleigh Prestige, 1980 Motobecane Grand Jubile, custom 531 track, and a bunch of tinker bikes of all type
Generally, cartridge refers to a sealed unit as far as I know. I think the name for what you are referring to is a "retainer." In any event, you pretty much answered your own question there: it's essentially an optional piece, but it does serve it's purpose. If you don't have one, and don't feel like going down to the shop to pick one up (depending on your standing with the shop, they might even just give you one), I'm sure the world won't end.
Personally, I use them, but I've never built up an old BB that didn't already have one.