Once an engine has been heat soaked, in other words everything is as warm as it is going to get there is no advantage in leaving it idle unless as Kevin points out the thing has the leans, the batteries are junk, or the driver just wants to stay comfortable.

Short hops that are over before everything warms up are bad for a coach because the engine has a lot of moisture not yet evaporated and it condenses which is not good for the engine. I don't know if anybody has noticed, but ever since the fuel prices went crazy a while back companies that never shut their engines down are doing it all the time now. The UPS driver is barely stopped in our driveway when he shuts it off, and he starts it back up 30 seconds later after putting the delivery in our garage.

I'll shut down for a 30 minute lunch break, but not a 5 minute bathroom break.