Surveillance was used, however, to identify and track the Boston Bombers, leading to death and capture, and parallel construction was used to create an unstoppable court case, leading to imprisonment and death sentencing.
But you are right - these tools are not used merely to detect and stop terrorist activity. It is also used to track and stop ideas, to give leads and to give extremely powerful intelligence to the FBI, CIA, DIA, NCTC, etc whenever it is needed for some purpose. To those in power these capabilities are extremely powerful.
The existence of these powers does not mean that they will be abused on a systematic level - though of course the potential is always there. One danger is that America's fight to remain relevant in a world that may be moving past it, it may turn ever more Fascistic. In this case, these are powers that would be extremely dangerous for the government to have.
But take a step back to think about this. As citizens we are worried about the use of these capabilities to thwart the public. They are so powerful and so complete that we fear being subjects of its application - viscerally.
To the extent that the good will and checks and balances have worked to keep the brunt of these capabilities aimed outside US borders there are people who do fear, and deserve to fear, the wrath of the US government - even those who might otherwise be innocents, bystanders, or casualties of the struggle for power.
My heart goes out to these people stuck in the middle and it beats faster at the thought of being one myself.