Sean does have a point. By the time you find your car missing, report it and the Police maybe finding it's signal, it could already be in a container, which no one will find. Yes they do have there merits if the 'wood be' thief comes calling but a professional team wouldn't let it worry them. I've found that you need one installed just to satisfy the Theft cover of your insurance policy. So I'll keep mine up to date just incase it does go walk about and at least I'm covered!
Check you insurance to see if they insist that you must have one installed and working. A Plan require a copy of the Tracker certificate.