Intelligence is one of the few rare fields based wholly upon the idea that the ends justify the means. There are no easy answers there.
If we judge the means by the ends, I do not believe that their end provides sufficient justification for their means. They appear to believe otherwise, however they fail to offer any evidence for their perspective; as an American, I am feeling ever more alienated from the organizations which were theoretically founded for our benefit.
Did it? Has it? Unknown.
The trouble with intelligence is that it's only effective when done with secrecy and fairly broad latitude to operate. There are few easy answers here.
In short, it sounds like you are advocating for an agency which can take arbitrary extralegal action at its own discretion, without providing reason or explanation, and without providing any demonstrable benefit to anybody, because it's secret.
Frankly, I find the idea terrifying. I understand that intelligence agencies need some quantity of secrecy and some degree of latitude. Like you have repeatedly stated, there are no easy answers. But that doesn't mean we shouldn't ask the question. What the hell are these people doing, and should we let them continue? What is growing in our intelligence sector -- is it an institution that will be found to have brought the world benefit, like Bletchley Park, or will it be seen to have become a thin facade over a malignant, self-interested organization, potentially culminating in something like a secret police?
I would argue that theoretically, a government (or other entity) could use intelligence but use it within a set of moral and/or ethical guidelines that uses a system of checks and balances.
Generally, intelligence cannot operate openly, even under a strict set of guidelines. Further, there will always be situations where efficacy runs into guidelines and something has to give. Would you be willing to violate the privacy of one person to prevent an attack that would kill five thousand? How about a dozen people's privacy? A hundred? A thousand? A million?
As I understand it, those aren't purely theoretical questions in the world of intelligence.
Can this claim be substantiated with evidence?
The idea that "spys gonna spy" is one we need to start collectively challenging. Why do we need these organisations at all? If NSA/GCHQ were wound up and their technical specialists re-allocated 80% to domestic law enforcement for computer forensics purposes, and 20% to a new dedicated counter-intel-only organisation, would the sky fall? I doubt it.
Why don't we skip the suggestive "thought experiments" and look at some facts instead.
A grand total of 3467 people in the USA have been killed by terror attacks since 1970[1].
In the same timeframe 2091 americans were killed by lightning strike[2] and roughly 102.000.000 died of old age.
Please explain how these numbers justify the NSA's yearly budget of $75 billion dollars, and their documented, ongoing violation of millions of people's privacy.
[1] http://www.start.umd.edu/gtd/search/Results.aspx?chart=fatal...
[2] http://en.wikipedia.org/wiki/Lightning_strike#Epidemiology
[3] http://money.cnn.com/2013/06/07/news/economy/nsa-surveillanc...
[4] https://firstlook.org/theintercept/2014/08/25/icreach-nsa-ci...