When to keep a secret

I was listening to a interview with Bruce Schneier at Columbia Law School on the NSA and all that (audio here). It’s well worth listening to — I think that Snowden’s whistleblowing is worth it just for all the talks and interviews Schneier has given as a result.
Schneier pointed to another positive consequence of the Snowden revelations: the NSA and its counterparts now know that whatever they do, there is a chance it will come out in the open. The implicit assumption that no one will ever find out what you’re doing makes people careless and makes them cross boundaries they wouldn’t cross otherwise.
It reminded me of something journalist and historian Timothy Garton Ash wrote following the WikiLeaks cables a few years ago. He said that anyone wanting to keep information secret, should be able to withstand the following test: if this piece of information became public, could you credibly explain why it should not have become public?
Indeed, few would have batted an eyelid if it turned out that the NSA only read the email of Al-Qaeda members and listened to the phone calls of Kim Jong-un.