The NSA prevents thousands of potential terrorist attacks each year...we just don't hear about it because it's classified information. Of course one or two will always slip through the cracks. No process is perfect.
And here is the problem with the unaccountability of the NSA: if not even the broad outline of its operational statistics is available for public scrutiny, and we can't trust the politicians who do have that data available (assuming it is), how can we have a meaningful public debate about this?