I've had a 340PD since December. Have it on right now as a matter of fact. I have never even stuck the key in the lock. After what I've read about it possibly locking up when it's not supposed to, I figure I'm not even going to activate it. I'm not an engineer (obviously) but I figure why tempt fate?
I'm considering installing the plug.
In answer to your question, when would I use the lock? Can't think of a time or reason.