Trials are secret because they don't want to reveal further secrets. On the other hand you hear people saying: "If you are doing everything right, you have nothing to hide."
The difference is that in a democracy you should be responsible for the actions of government and you need to be able to audit governmental action, not the other way around.
I am surprised that they really have zero cases where mass surveillance helped. I would have thought they would make some up at least, but that got probably too hot.
Of course there is a massive amount of secret cases where it did help, but they just cannot tell you about...
It is a bad excuse that can be used to justify anything. Instead the case for secrecy needs justification.
