Again, nothing you've offered as a benefit actually does anything that ordinary police work couldn't do just as well. Besides the unnecessary cost of maintaining such a system, it also opens up some sticky issues:
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Who exactly is reviewing these tapes? Is there a police officer sitting at a desk with a huge CCTV display in front of him? If so, the very act of watching those live feeds can constitute an invasion of privacy. And without full disclosure of EVERYTHING (which is unacceptable as having every step of my life released into public domain isn't acceptable either) you really don't know who's watching.
Saying that you can't prove the system
is abused is a very different thing than proving it isn't. Even if documented examples don't exist, that doesn't mean that we can trust that, for all time, some abuse won't take place. What if the system in question was being set up in Moscow, a city KNOWN for its police corruption? That would be bad on a number of levels, as you're giving clandestine survalence capabilities to an organization that can't be trusted without them.
If you're willing to dismiss 1) and 2), you've still got the issue of accountability for the police or whoever monitors the system. Who do they report to, and how? If they report to any form of government, there again is another avenue for abuse. You, for whatever reason, think government can and should be trusted to not make abuses on "good faith" and for that I'm sorry, because no one can be trusted in that way, most especially those in power. And you can't say they would be accountable to the public, again, because there's no way that the public would actually get to know how and for what ends the system was being used and have it still function at all.[/list]
I will also reiterate that the real problem with a city- or nation-wide survalence system is not the legitimate uses, it's the potential illegitimate ones. And you can't counter those concerns by offering more legitimate reasons.