this one is almost directly related to the issue. To save you the 70 page read, the summary: it concerns diesel electric generating plants in rural Alaska, in which mechanical problems relating to running at part load are studied. Many problems are identified, mostly stemming from hydrocarbon buildup and glazed cylinder walls. The hydrocarbon buildup results in plugged injectors, sticking rings, valve sealing problems; all of which can shorten engine life. The cause was considered to be delayed ignition timing due to lower operating temperatures and smaller injection charge. This delayed ignition timing was thought to have several detrimental effects as the cylinder was already in its expansion cycle. It was thought that if injection timing could be advanced for operation at part loads (as electronic systems do) that much of the problem would be alleviated, however these engines (and most small boat diesels) have fixed timing. Several possible solutions were studied but none were particularly satisfactory. The engine load considered to avoid these problems was not a perfect consensus, but 60-80% seemed to be the numbers most often mentioned.

Now there are a few confounding details when relating this to boat engines. This paper is from 1985, the engines must be that old or older. In the paper, it is mentioned that there have been several advances in combustion chamber and injector design which help. Further, these engines are all 1800 or 1200 rpm engines, and as they are running AC generators they must be run at that rpm even at part load. This is distinct from a sailboat auxiliary, in which rpm would drop with reduced load. That has a direct effect on ignition timing: at lower rpm there is more time for the fuel to heat and begin burning.

By one person's estimate in the report, engines were lasting 10,000 hours which might have lasted 20,000 MTBO, if they could have been run close to full load.

So there is at least a plausible explanation of a problem. Can anybody find a research paper more directly related to boat engines?

http://dot.alaska.gov/stwddes/research/assets/pdf/ak_rd_86_01.pdf