There was a time, when diesel engines were hard to start because of engine design, available starters, batteries etc. , when if you got one running you sure didn't want to shut it off till you were done with it! Also, with big trucks, often the engine in the old beast is the furnace and electrical system so you had to leave them running for your comfort. Most people observed this and, I guess think it's cool or something and leave their diesel pickups running while they go shopping, have breakfast ,whatever. It is hilarious to go to Walmart in the summer and listen to all those pickups idling away in the parking lot, I guess keeping the cab cool for the owner!
Yes, a diesel can be left Idle with less concern than an old carburated gas, but you are still just needlessly wearing it out and getting zero miles per gallon! why do it? True, with a turbocharged engine you should let it cool a few moments before shutting it off after a hard pull because the turbo gets so hot the oil will "coke" in the bearings leading to failure. That too was more of a problem on the old diesels but still somewhat true.
Ed