Paul,
Those aren't for building, those are talismans. It's kind of a Murphy's Law thing: if you have extra ribs, you never break anything in the wing and therefore have no need to replace them. Because you already have them. I think they only put those in kits made on Fridays?
On second thought, maybe they are there for when the plane just isn't working right and you can "toss the ribs," sort of like tossing the bones to get the demystifying answer from the Stunt God(s)? I don't know if it works because I never got any extra ribs. I did get some extra sheeting in one kit that I couldn't figure out where to install. You can't toss that, so I just scabbed it on the wing as extra planking. It was a pretty sheety plane in the end.
The Divot