The fact that the Boston bombings still happened should be proof enough that these systems either A) Do not work or B) are not being used against this type of threat.
If it isn't in defense of incidents of domestic terror, what the fuck is it for?
Terrorism is a small subset of what the intelligence community supports. The majority of intelligence collection is used for strategic military planning and political decision making. See:
http://en.wikipedia.org/wiki/United_States_Intelligence_Comm...
Maybe. (This is normally "bear-repellent amulet" type thinking, but I think there's a specific exemption in this case, in that there are good reasons why you'd see no evidence of the system's effectiveness: the government doesn't like to get the public riled up by terrorist threats--terrorism is entirely about riling-up, after all--and so, when the terrorism is caught ahead of time, you'd imagine they'd sweep it under the rug. I might be wrong, though; this still might be a bad justification!)
I'm quite sure that's exactly what the government likes to do. That's the very reason that "terrorism" is in the headlines as much as it is: it serves the government by allowing them to extend their powers and budgets enormously. Feed scary stories about "terrorism" to an uncritical public and they'll beg you to take away their "freedom" in exchange for greater "security".
http://www.youtube.com/watch?v=3lDZjiPok3w
Yes, it's a pointed example ("TERRIFIC ACTS OF ATROCITY!!!!1111"), but if you're going to tell me that "terrorism is entirely about riling-up", and therefore politics and powermongering aren't, I think pretty much all of known, including recent, history utterly destroys that argument.
Because it was happening all the time before