Here's a reading (MP3) of a recent Guardian column, 'Cybersecurity' begins with integrity, not surveillance, in which I suggest that the reason to oppose mass surveillance is independent of whether it "works" or not -- the reason to oppose mass surveillance is that mass surveillance is an inherently immoral act:
The Washington Post journalist Barton Gellman and I presented an introductory session at SXSW before Edward Snowden's appearance, and he made a thought-provoking comparison between surveillance and torture. Some of the opponents of torture argue against it on the ground that torture produces low-quality intelligence. If you torture someone long enough, you can probably get him to admit to anything, but that's exactly why evidence from torture isn't useful.
But Gellman pointed out that there are circumstances in which torture almost certainly would work. If you have a locked safe – or a locked phone – and you want to get the combination out of someone, all you need is some wire-cutters, a branding iron, some pliers, and a howling void where your conscience should be.
The "instrumental" argument against torture – that it doesn't work – invites the conclusion that on those occasions where torture would work, there's nothing wrong with using it. But the primary reason not to torture isn't its efficacy or lack thereof: it's that torture is barbaric. It is immoral. It is wrong. It rots societies from the inside out.