This is more or less a justification for any action the NSA might take.
They already have access to pretty much *all* communications in the world. I for one am sure glad that helped prevent the Boston bombings and the recent attack on the mall in Kenya.
If they are already unable to detect and prevent bad things from happening at the hands of terrorists, what justifies attempting to crack one of the few means of privacy we have left? In their rampant pursuit of obtaining *all* communications they have trampled the rights of individuals to any shred of privacy - and apparently accomplished absolutely nothing of major value before it happened. Sure, the ability to subvert communications world wide might let them track down a terrorist leader a decade or so later but is that enough justification for crushing the rights of every human on the fucking planet?
They used to do this stuff using human assets - actual members of the CIA going out and recruiting agents, analyzing data received, finding targets and then determining what to do about them, but when they came across the absolute "sexiness" of electronic spying, they cut waaaaaaay back on human spying, turned the problem over the NSA and cut the budget (more likely spend more on the NSA than they did on CIA employees and bribes to prospective agents). In the process they apparently decided it was necessary to spy on all American citizens as well, in violation of the law, as well as on all the citizens of their friends and allies.
I hope they have been unable to crack TOR, even though I don't use it, because its one of the few options people have for privacy, and I have yet to hear them provide any details on anything they have concerning terrorists actually using this technology.