I was kind of surprised by the Delta talks. Always seemed like it would be a USAir-United merger.
The problem for United is that it has little presence in the east; only Dulles is a true east-coast hub. They're covered well in the west with Denver, San Fran and LAX (and Sea-Tac, to a lesser extent), which explains their significant presence on the trans-Pacific routes to Sydney, Narita, Hong Kong, etc.
Adding Atlanta as a second hub would provide a better foothold in the east; still, it wouldn't be nearly as advantageous as gaining USAir's hubs in Charlotte and Philly, IMO.