It's a truism that government secrecy is less about safeguarding national security than about covering asses: During the cold war it happened time and again that leaked intelligence secrets revealed not just something that our government knew about the Soviets but that the Soviets knew we knew — or vice versa. (The secret bombing of Cambodia, for example, could hardly have been much of a secret from the people the bombs were landing on; the only people in the dark, as always, were the American public.)
A vast amount is also classified through sheer inertia or habit. During the year I held a top secret security clearance, while working for the U.S. Congress, I can recall among the thousands of pages of classified materials I read a single piece of information that was a genuine secret, in the sense that it could plausibly harm American national security were it revealed; it had to do with a very specific capability of a weapons system then under development. Even the compartmentalized "codeword" intelligence material I saw (classified up the wazoo on the grounds that it could give away the capabilities of our intelligence collections systems) I very much doubt would have been any surprise to the Soviets.
During the Nixon Administration, more notoriously, secrecy invoked in the name of national security was used to hide political dirty tricks, domestic espionage on the White House's "enemies," and other sordid crimes.
More generally, though, Watergate struck a healthy blow against the entire notion (still much beloved by Dick Cheney, for one) that efficient government requires an umbrella of secrecy over internal policy deliberations so that decisionmakers may receive candid advice: a notion that, after all, goes against the very grain of democracy and the principle that the people have not just a right but a responsibility to be informed participants in the policy decisions made in their name.
There is no doubt that the latest WikiLeaks dump, of a quarter of a million State Department e-mail messages, is news. But if the aim was to strike another salutary blow at needless government secrecy, it makes about the worst possible case for a cause I for one am otherwise in much sympathy with. At first blush there is no whistleblowing here, no revelations of wrongdoing or corruption or rogue espionage plots or military adventurism, not even much in the way of history or insight into decisionmaking: all there is is indiscretion. And diplomatic channels are one of the very few realms of government where a case for across-the-board confidentiality actually has merit: wars have been triggered by diplomatic indiscretions less indiscreet than those contained in these leaked cables.
Many other commentators have already noted the grave damage that has been done to the ability of American diplomats to receive honest and accurate information about matters of the first importance in maintaining international security, promoting American values of human rights and democracy, and averting war; most stomach-churning of all is the thought that dissidents and human-rights activists in oppressive regimes who courageously took enormous risks in speaking to American contacts could find their courage rewarded with imprisonment, or worse.
In my recent talk about secrecy and intelligence history I quoted the observation that the government's obsession with secrecy promotes the public's obsession with conspiracy; as an observer both within and without the "black" world I'd also note that excessive secrecy breeds contempt for the entire system of secrecy: classification of the trivial makes it harder to protect the classification of the vital.
Steven Aftergood at Secrecy News has an insightful article on the state of government secrecy.