Joe Klein has a hunch:
The runoff is a probably a sure loser: most observers figure that Karzai--who is Pashtun royalty--will win a head-to-head contest with Abdullah, who is of mixed parentage, half Pashtun, half Tajik. On the other hand, if Abdullah presses the runoff and stays away from the Karzai government, he'll be able to go into the next election untainted by association with the Karzai regime.
On the third hand, if the U.S. doesn't stick around, there may not be another election, which is where Rahm Emanuel's threat comes in. It seems clear--to me, at least--that the preferred U.S. outcome is no runoff election (which would be yet another security headache for U.S. troops in any case) and a coalition government in which Abdullah negotiates and achieves a significant portfolio.