I'm sure they have plenty of proof ready to share with the public, to prove that the system is worth it, even though it's unconstitutional, but apparently no one cares about that and about American values anymore:

http://www.youtube.com/watch?v=Etm4dHdaApo

Or to explain that and how they stopped a threat might reveal how much they know, who their informers are, what type of codes they can break, and hence make it hard to stop the next one. If you tell Bad Guys™ (a) that and (b) how you stopped a previous attack, then the next set of Bad Guy™ will know not to make those mistakes.

"They" knew that the "Ft Hood shooter" had been in contact with extremists, yet we all know what the end result there was.

    13 dead
    30 + wounded

This is extremely counterproductive. You aren't going to get anywhere simultaneously bemoaning the damage caused by extremist acts and the excesses of the NSA.

When you point out the failures of the NSA to stop specific instances of terrorism, it only suggests that they need more resources and legal surveillance. The reasons for wanting the NSA surveillance to stop have to be that the innumerable injustices suffered by citizens under a surveillance state would be worse than the occasional extremist attack that would be preventable under such surveillance.

No, that's not what he is saying at all. With the maddening amount of resources ($80 billion for fuck's sake) spent on surveillance of both foreign and domestic communications, incidents where the agencies have the resources and intelligence to stop attacks but don't/can't shows that no amount of money spent or rights lost will give us absolute safety.

Yeah, it's weird what our existing civil liberties are able to prevent out intelligence agencies from doing.

If it were the Stasi, Gestapo, or NKVD that noticed an Army major communicating with a Resistance element for any reason you'd have been able to count the remainder of that major's life in hours. FBI, on the other hand, went "Meh, not enough evidence to start an investigation".

So be careful about the conclusion you draw from that. All you've really said is that we currently have handcuffs too tight on the NSA and FBI.

I don't think so. Any BS success they would advertise to high heavens.

Well ... the day after Osama was killed it was all in the news and all positive. If they stopped something on similar scale they would not have reasons to hide it.

Which is the same nasty logic as if you have nothing to hide routine.