Re: What we've learned...
No. What we've learned is that there are some in the population who have bought the whole "the world is a REEEEEEAAAALLLYY dangerous place" story, and will excuse the State anything in the pursuit of perpetrators.
I'd rather have the small risk of something bad happening. Let's face it, the sort of crime most of us* will ever come across is not the sort that this kind of intelligence will ever pick up - petty vandalism, picked pockets, burglary, minor offences against the person. By all means, if the police have a suspicion that there is a conspiracy to commit a major crime, then they should do their jobs as they always have - work out who is likely to be involved, persuade a judge to give a warrant for proportionate intelligence gathering, and then, in the event of an arrest, gather sufficient evidence to persuade a jury that the suspects actually did it. That is the rule of law. Treating everyone as a suspect simply for being alive is wrong.
* and the overwhelming majority of us will never experience any of these anyway