Codeshare Partners vs. Skyteam Members
 

As I was thinking about the impending codeshare relationship between GOL and DL, the thought occurred to me that a codeshare partner can be more valuable than a ST partner. I will hopefully be able to buy a DL ticket that connects to a GOL operated flight with a through fare. The codeshare flights will earn status miles.

Contrast that with a ST member like UX that so far does not seem to have any codeshares with DL. I may connect to a UX flight in MAD or BCN, but it would seem that the UX flight would be a separate ticket.

Now ultimately, I hope GOL joins ST, but as long as I can fly to their destinations on a single ticket/fare and earn MQMs, I don't see that ST membership is that crucial.

Another example... I have been using AS flights for years, earning DL MQMs and miles even though they are not ST members.

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.