Not only does code-sharing reduce competition, but frequent flyer benefits will definitely be hurt for SkyMiles/OnePass/WorldPerks members.

Even if award levels remain the same (and I am sure they aren't, I & D business class tickets are getting harder to score as NWA is making more seats available as Z class "sale" seats), more people having more access to award seats means that less seats are going to be available for you.

And even though I've heard Continental's Business First is pretty good, I would prefer to spend my WorldPerks miles on World Business Class and my (much more slowly accumulating) SkyMiles on Delta's products.

And don't even get me started on the point where they start to "equalize" benefits between programs (where awards get "enhanced" to cost more or they do away with unlimited Elite upgrades).