First of all the one hour per month requirement is right out of the Yanmar operators manual. Beyond that I have spent my entire life working as a mechanic. Well that is not entirely true as for the past 10 years or so most of the work done in our shop is done by my employees.....
My field is not marine but I have sailed and sailed hard for the past 20 years. During that time I have learned a lot about maintaining a sailboat. There are however a couple of observations I have made even working on the light duty diesels I have maintained in my life.

1) Even the VW diesels I have maintained are much happier when driven at higher speeds regularly. I have replaced something in the order of 50 cylinder head gaskets on early VW Rabbit diesels and you knew exactly how each engine was driven based on what you saw when you took it apart. Cars driven primarily around town always had a ton more carbon build up in the ports and in fact the longest lasting highest mileage engines always came from cars that commuted regularly on the highway. I replaced a number of VW Diesels that where low mileage engines that where just worn out in 100,000 miles of around town driving. Worn bores, heavy oil use, low compression etc. These were engines that I maintained and saw proper oil service and general maintenance. My mother in law has a 2000 VW Diesel that is driven only locally. It is very low mileage and it is a carbon mess! Whenever I get a chance I take it out for a hard ride and it just runs better! FWIW there is nothing wrong with the car. It just doesn't see the loads or drive time necessary to clean up the engine. Remember that the newer the (automobile diesel) engine the more likely the engine fuel system is managed by a computer with common rail fuel system. This is an entirely different can of worms! Just as we had huge leaps and bounds in gasoline engine reliability over the long haul with the introduction of fuel injection we are now seeing diesel engines that are cleaner and more able do go high mileage with common rail diesels. I am certain this impacts how the engine lasts.

2) Most diesel engines I see on both sailboats and small cars and trucks are not maintained all that well. There is more to maintaining an engine than changing oil. Valve clearances need to be set, injectors removed and checked/popped (at 500 hours on small Yanmar's), cooling systems flushed, crankcase ventilation parts replaced on schedule on newer motors, water pumps replaced etc. I have a good friend who worked for years at Stanadyne on diesel injection systems. He is emphatic that the best way to kill a diesel fuel transfer pump is to run the engine without a lubricity additive. I typically use the Stanadyne product and I have yet to have a customer who follows this advice need a pump. I have seen lots of pumps fail..... I guess I am saying that there are many enemies of small diesel auxiliary engines not just "engine use habits".

I remember listening to a seminar where the explanation of why diesels need to operate at a load was given. I have no recollection of the "why" now but I will ask my friend for a quick re-education. When I get it I will post on the forum what I re-learn.

Bruce