I am sure NW and CO don't want Song included. They don't want people earning "equal" FF miles on a low cost carrier, and being able to redeem them for flights on their high-priced flights. Nor do they want Delta to have the advantage of being able to fulfill their half of the bargain (i.e., people with miles earned on CO and NW flights seeking partner awards) by fulfilling Delta's end of the bargain with low cost Song flights.

So, look to the partners. I am sure Delta would love Song to participate in the alliance, since it would only benefit Delta.

Djlawman