It's a deterrent, and is better than nothing. Most thieves will grab and walk/run away, and generally aren't smart enough to do anything more.
Many (most?) common locks are "insecure" against knowledgeable thieves, but that doesn't mean people should stop using them. (Hint: a knowledgeable teenager can probably open your home's front door lock in under a minute -- and possibly in seconds -- using information found on youtube, and some easily made tools.)
Better than nothing is to not leave your system somewhere out of your view. I'm surprised by all the stories of people leaving systems on desks in libraries and coffee shops. I didn't even know that MBPs had locks on them when I ordered my rMBP.