I think the test should be whether the document has moral or just tactical significance. To give examples from the article:

Guantanamo Bay interrogation manual? Moral significance; leak.

Possible US use of chemical weapons? Moral significance; leak.

US order of battle for 2009? No moral significance; don't leak.

And in the interest of fairness:

Al-Qaida interrogation manual? Leak.

We need a picture of both/all sides' dirty laundry, and Wikileaks doesn't really give that.