There is no need for airlines to code-share in order to get a single ticket including both of them on the same itinerary. They need to interline - all alliance partners do AFAIK - and they need to publish fares that permit the use of the other carrier for the flights in question, but there is nothing specific to a codesharing arrangement that makes that particularly better.
Delta does publish fares to cities in Spain that use UX as connections. NYC-BIO is one that I found but I'm sure that there are plenty others. Just look up whatever cities UX serves. It might be cheaper to stay on AF metal/codes and connect in CDG because of the way they've structured the anti-trust immunity deal - those routings are also more profitable for the "main" carrier - but the UX option exists today.