Privacy and security are almost never a zero sum game. In this case, reducing privacy isn't going to help find more 'criminal/terrorist activity'; It will just cause them to use Freenet, TOR, steganography, for comunication etc. instead and result in making it even harder to track real criminal activity.
Secondly, common people are really really bad at making these risk-reward trade-offs (for instance, many people have a fear of flying, but a more rational reaction would be to have a fear of travelling to get a flight as you're more likely to get killed in a car/bus on the way to your flight, than actually flying; you may tell your children to 'never talk to strangers', but in fact that would put them in a far worse position if they ever got lost -- the huge majority of people are not evil! etc.) - we'd be better off delegating to a panel of economists and statisticians to determine the outcome.