This talk by Dan Geer explains the NSA mindset of "collect everything": I previously worked for a data protection company. Our product was, and I believe still is, the most thorough on the market. By "thorough" I mean the dictionary definition, "careful about doing something in an accurate and exact way." To this end, installing our product instrumented every system call on the target machine.
Data did not and could not move in any sense of the word "move" without detection. Every data operation was caught and monitored. It was total surveillance data protection. Its customers were companies that don't accept half-measures. What made this product stick out was that very thoroughness, but here is the point: Unless you fully instrument your data handling, it is not possible for you to say what did not happen. With total surveillance, and total surveillance alone, it is possible to treat the absence of evidence as the evidence of absence. Only when you know everything that *did* happen with your data can you say what did *not* happen with your data.

The link for this article located at Schneier on Security is no longer available.