I could see the advantages for an airline years ago as a way to expand their destinations and feed their long-haul routes.
But now what's the benefits of BIO-LHR-ORD-MCI (IB-BA-AA operated) to be BA-BA-BA marketed? Oneworld partners have interline e-ticketing, interline check-in & baggage handling.. any OW-ticket holder (business&first) will have access to VIP lounges at any of the 4 airports and a FF Iberia Plus member would earn the same points being marketed as IB, BA, AA or even QF!!!
On the fares side it's also remarkable the tiny diferences in price. MAD-JFK on AA (c/s) is ussually less than 2€ cheaper than on IB (operated) and i bet that's only because of currency changes.
There's only one advantage i think... Serviberia and iberia.com can only sell IB coded flights but that would be bypassed by adding all OW routes to their selling system and they would get the process fee instead of any other travel agency.
So then, why do you think c/s between alliance members are still on?