While I think a "a la carte" Kit Of Parts is an interesting
concept, implementing it in the real world would just be a nightmare for FIRST and teams. Just like in engineering a mechanism, the more points you create where something can go wrong, the more likely it is that something will go wrong. K.I.S.S. doesn't just apply to robots.
Even if our team don't use parts from a particular KoP that particular, we just keep the parts for prototyping for the next season. This year, our team went through the equivalent of 2 Kitbot chassis (which we have never used) in prototyping ideas. We still keep all our Victor 883 speed controllers from older robots on hand, as they still work for prototyping and/or low-current mechanisms on practice robots. Just because you may not use every component in the KoP every year doesn't mean it won't find a useful purpose at some point in time.
Or if your team has these extra parts, and is in a graciously professional mood, why not donate them to a new rookie team to help get their team started? Surely, for a rookie team who doesn't have a huge (or at least modest) "archive" of parts on hand that established teams do, any additional parts for them would be a Godsend.
Rather than FIRST [try to] overly complicate the Kit of Parts, why not have them officially "endorse" something like CD-Swap for the spare parts from Kit that teams are willing to donate/trade to other teams. It would take a lot less manpower on their behalf (which saves $$$), and would only strengthen the program
while building gracious professionalism. Two birds, one stone.