Here Mr. Terrorist
Some researchers in London say that tagging every passenger in airports with RFID could help fight terrorism. Well, they could give everybody some chewing gum, and that might help fight terrorism, too, and quite frankly, this plan sounds about as useful.
Without the device somehow being locked to or implanted in people, any potential ne'er-do-well could simply ditch it, or attach it to someone else. Never mind the problem many people would have with this type of surveillance for a moment, and focus instead on the sheer uselessness of the system: it won't stop anybody from actually doing anything dangerous or criminal, it just means that if a person's been flagged as suspicious, they can be identified, and the system can follow them, and then they do something, it might get caught on camera.