So it appears something is bugged with the auto logout due to inactivity.
I may just be stupid as hell, but stick with me for a moment.
I was farming the instaspawn pirates in STV, got bored and left WoW open. I came back to a single pirate beating on me and my chat logged filled with the message "You have been inactive for some time and will be logged out of the game. If you wish to remain logged in, hit the cancel button.". I clearly wasn't logged out.
As far as I knew, even if you were in combat you'd be logged out before, but apparently this is bugged.
I'm assuming you could beat on a dummy in Orgrimmar and not get logged out. Other than spamming chat easier without getting logged out, I can only thing of a few real benefits to this, but maybe someone else will think of something.
tl;dr - being in combat prevents you from being logged out due to inactivity (this isn't intended as far as I know, if it is, ops)