I understand the point you are making. But I think the perception here is important as well. And even though the probability of someone opening the door is quite slim people still lock doors anyway. Irrationality plays into this. People lock bathroom doors even though only part of the time they are exposed in any way shape or form. Most of the time they are only sitting down on the toilet seat (or if a many perhaps standing for a brief period).
Personally on the one hand I agree with you and rationally it isn't a credible threat. But what if I am away on vacation and the door is unlocked during that week that I am away and I don't know about it? How comfortable do I feel with my office door being unlocked for a week? It's a small chance of a big thing happening. (Even if 99.99%).
J. Random Hacker from Waziristan hacks your account at the lock company.
So? He is _there_ and you are _here_. Annoying but not fatal.
Unless you're worried that he has a cousin in town?