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.