jugimaster said "How many offices contain people that could cause some real harm if they found an unlocked computer?"
Are you certain that this is impossible in your office? It could be that a disgruntled (former) cow-orker might wander by your office and see your computer unlocked and be tempted to take out his frustrations using your access.
Also, many workplaces with larger numbers of employees do not really have any truly effective ways of preventing someone from being able to wander freely, if only for a few minutes. Sometimes, this is even possible in places like hospitals.
Back in school, while studying network security, we did a lesson that involved the class being turned loose on the campus to see what we could get our hands on. We also were to try to figure out what places we might be able to sneak around in, or talk our way into. We did things like dummying up official looking work orders to "install updates", using a staff roster that we found on the school's website, in order to get access to computers. We were also able to slip into a lot of offices with unlocked workstations. One guy happened to have a hat with the phone company logo on it and managed to talk somebody with more keys than sense into letting him into the server room while the IT guys were out of the office.
Part of the lesson learned was that unless you take measures to make sure that people can't get access to your workstations, starting with locking them, you can't guarantee anything about your organization's security.