It is hard to see Virgin operate without Branson at the helm. The airline's perks and amenities, from its complimentary chauffeured pickups for upper class passengers to generous baggage allowances for passengers in coach, fits with Branson's whimsical disregard for convention which has made Virgin such a hit with its fiercely loyal customers. Without Branson at the helm, Virgin might as well just liquidate.
Branson has said that Virgin would probably need to join one of the big airline alliances in the wake of BA's takeover of BMI, but that doesn't mean he is about to give up control of the airline – let alone just give away landing slots to some Americans from Atlanta. Given all this, it would be best if Delta just invites Virgin to join Skyteam and saves its money for other things, like maintaining its new refinery or for buying some badly needed new jets.