Would you be willing to violate the privacy of one person to prevent an attack that would kill five thousand?
Why don't we skip the suggestive "thought experiments" and look at some facts instead.
A grand total of 3467 people in the USA have been killed by terror attacks since 1970[1].
In the same timeframe 2091 americans were killed by lightning strike[2] and roughly 102.000.000 died of old age.
Please explain how these numbers justify the NSA's yearly budget of $75 billion dollars, and their documented, ongoing violation of millions of people's privacy.