From Talking Philosophy:
One argument in favor of the leak is the classic Gadfly Argument (named in honor of Socrates because of his claim to the role of the gadfly to the city of Athens). The gist of the argument is that the people in government need to be watched and criticized so as to decrease the likelihood that they will conduct and conceal misdeeds in shadows and silence.
Given that governments have an extensive track record of misdeeds, it certainly makes sense to be concerned about what the folks running the show might really be doing under the cloak of secrecy and national security. If it is assumed that being part of the government does not exempt these people from moral accountability, then it would seem to follow that leaking their misdeeds is, in general, a morally acceptable action. After all, it would seem to be rather absurd to argue that people have a moral right to keep their misdeeds a secret.