"In practice, the best we can hope for is that, based on the best available information, there is some known probability that the message will be part of a terrorist plot. If that probability is less than 100%, we’ll be comfortable allowing eavesdropping on that message. If the probability is infinitesimal, we won’t allow eavesdropping. Somewhere in the middle there is a threshold probability, just high enough that we’re willing to allow eavesdropping. We’ll make the decision by weighing the potential benefit of hearing the bad guys’ conversations, against the costs and harms imposed by wiretapping, in light of the probability that we’ll overhear real bad guys. The key point here is that even the best wiretap policy will sometimes listen in on innocent people...
The drawbacks of wiretapping come in several flavors:
(1) Cost: Wiretapping costs money.
(2) Mission Creep: The scope of wiretapping programs (arguably) tends to increase over time, so today’s reasonable, well-balanced program will lead to tomorrow’s overreach.
(3) Abuse: Wiretaps can be (and have been) misused, by improperly spying on innocent people such as political opponents of the wiretappers, and by misusing information gleaned from wiretaps.
(4) Privacy Threat: Ordinary citizens will feel less comfortable and will feel compelled to speak more cautiously, due to the knowledge that wiretappers might be listening.
Cheap, high capacity storage reduces the first drawback (cost) but increases all the others. The risk of abuse seems particularly serious. If government stores everything from now on, corrupt government officials, especially a few years down the road, will have tremendous power to peer into the lives of people they don’t like.
This risk is reason enough to insist that recording be limited, and that there be procedural safeguards against overzealous recording."