On compelled testimony and preventative arrests

I have to admit, I am actually baffled as to who is right in today's debate over two controversial provisions of Canadian anti-terrorism legislation. On the one hand, the Conservatives are claiming that the ability to compel testimony and perform preventative arrests is crucial in the prevention of terrorism. On the other hand, the Liberals are saying that these provisions endanger civil liberties. But they've never once been used in the five years they've been on the books in Canada. So both positions seem a little empty to me. I am open to comments, but I'd like to hear especially why we should keep/repeal a law which doesn't seem to do any harm or any good.