Regarding idle vs shutdown.. I've always heard it the opposite. Idling is bad for engines, whereas the oil film will stay on the parts for a long time so you won't have "dry start" after a short stop. I think the statement "startup is the hardest on engines" is referring to cold-start after sitting overnight. Of course, the same argument goes for idling... "idling is bad" probably refers to long periods of idle, like leaving your car running while you run into the store to grab a snack or waiting outside the house for a female...
(said in good humor, of course).
So, I did a quick google search and found this (not that it's authoritative, but it makes sense):
Engine wear
Idling damages your engine components ...
...including the cylinders, spark plugs and the exhaust system. When an engine idles, fuel residue builds up on the cylinders, spark plugs and exhaust system.
An idling engine is not operating at peak temperature, resulting in incomplete fuel combustion. Fuel residues can condense on cylinder walls, contaminate oil and damage engine components. With more engine idling these residues tend to deposit on spark plugs. The resulting plug fouling can increase fuel consumption by 4 to 5 percent. Excessive idling can also cause water to condense in the vehicle's exhaust. This can lead to corrosion and reduce the life of the exhaust system.
Residue from idling leads to costly damage and decreases the car’s gas mileage. Idling is bad for the engine. On the other hand, frequent restarting has little impact on engine components such as the starter motor and the battery.
It probably doesn't matter either way. But there seems to be a murphy's law or some quirk of the universe that goes like this: "The likelihood of your engine NOT starting is directly related to the urgency behind NEEDING it to start." We learn this principle from watching horror movies.
I've always had an irrational fear of shutting off a running vehicle in traffic for fear it suddenly and inexplicably will refuse to start up again.... lol
