That depends on what he used the e-mail address for. If it was strictly for romantic liaisons, it's no big deal. But if he used it for any other purpose (talking with his lawyer, chatting with senators, etc.), it would be a hell of a platform for social engineering attacks. In that scenario, the information in Petraeus' personal inbox is beside the point[1]; you can use the trusted address to get your hooks into something more interesting.

[1] Ignoring the blackmail value of the affair.