For the people who are saying that this is paranoia, say that after you've had something of value stolen from you. I've never had a bike stolen, but I did have someone break into my college apartment once (when I wasn't home), and take a bunch of camera equipment and an electric razor. The razor I wasn't concerned about because I didn't use it, but the camera equipment pissed me off.

But worst of all was the feeling of being violated in some way - of someone else having that much control over me that they could just take my stuff and me be powerless to stop them. Even if your bike is a beater, if you like it, and have enjoyed riding it, and rely on it for transportation, you will be super-upset if it gets stolen. Until you know that feeling, you should lay off on calling people paranoid that are advocating common sense.