It's a good question. While everyone is crowing about this "sex scandal" being a side show - it may be one of the linchpins to an actual crime - blackmail, obstruction, perjury, and more...
If we put on our "Chicago thug hats" for a moment - threatening a guy like Gen. Petraeus probably won't get you very much cooperation. He's seen human behavior about as vicious as anything in the last millennium. So, what does a thug do to make a guy like this cooperate? Snatch someone he cares about who is innocent of anything related to the crimes and threaten their destruction if he doesn't cooperate...