If everyone knows they are watched, will the behave?
It has effect if the police car there is sitting in a speed trap.
People are more polite if the phone conversation is recorded.
But not always.
The trouble continues to be so many false positive and not enough people to follow up on them.
The analogy is the every one speeds on the highway.
Imagine keywords, website hits, phone triggers resulting in millions of leads.
Despite PRISM being in place, the NSA did not catch the Boston Bombers until after the fact despite extensive reacher on bombs via the internet.
None of the recent jailbreaks were detected in advance.
The theory is the people who are safe will be safe and the stupid will always be caught. The others just adapt and the agencies only react after the fact.