While there are various legal concerns regarding these documents, my main concern is with the ethics of this leaking. I will consider various arguments in the course of the discussion.
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.
The obvious reply to the Gadfly Argument is that even if it is granted, it does not cover all of the leaked material. After all, not all of the material deals with moral questionable activities that should be thus exposed to the light of day. As such, more would be needed to justify such a leak.
A second obvious argument is based on the assumption that in a democracy the citizens have a moral right to know what the folks in the government are doing in their name. This right can be based on the idea that the citizens are collectively responsible for the actions of their government and hence have a right (and need) to know what is actually going on. This right could also be based on the notion that the citizens need to be properly informed so as to make decisions. Since power comes from the people, one might argue that the people have a right to know about how that power is exercised and the information to (in theory) exercise it wisely.