There is no science behind that recommendation, just my experience for elevating oil and combustion gas temps and the effect that has on internal engine components, much of which is based on my review of oil analysis reports. But again, it's anecdotal.
A long run of hours at higher load is almost certainly more beneficial, it's just most cruisers avoid that because of the cost, so the periodic short runs is a more palatable alternative.
Ours is a combination of anecdotal and experience by professionals with manufacturers' recommendations.
Steve reemphasized proper break in. Well, in my opinion. what you're told for break in also remains good to consider beyond. I believe in varying speed. Also understand we're running planing and semi-planing and not engines designed for slow speeds and constant running. I don't believe in long sustained runs at WOT. We limit WOT typically to 15 minutes and no more than 30. Many manufacturers recommend brief runs at full cruising speeds of up to 80% load. I've heard experienced captains and engineers say 15 minutes every four hours and 30 minutes for six hours and 15 for six and various other combinations. No one knows the perfect numbers. Nor is 80% some perfectly defined number. Actually, it's my opinion they often chose 80% to mean "not above" as in not WOT. We go with ranges based on what we've determined to be our ideal high cruising speeds. And where do those speeds come from? Well, looking at our performance curves and seeing where fuel usage changes significantly. I look at various boats and here are examples.
Boat 1, between 65% and 85% load, uses 33% more fuel for 8% more speed so cruise at around 65-70%.
Boat 2, between 60% and 85% load, uses 37% more fuel for 14% more speed so cruise at 60-65%.
Boat 3, between 30% and 75% load, the nm/gal stays constant. Between 75% and WOT uses 30% more fuel for 13% more speed so cruise around 75% load.
Boat 4, between 67% and 84% load, uses 25% more fuel for 11% more speed, so cruise at 65-70%.
Boat 5, between 50% and 70% load, uses 40% more fuel for 18% more speed. Between 70% and WOT, uses 42% more fuel for 20% more speed. 50% is clearly the sweet spot, but because we like a bit faster and are still well under the 80% number the engine manufacturer recommends as a max cruising, we generally run between 50% and 70%. Now, that boat is high performance and 50% is 38 knots while 70% is 45 knots. WOT is 54 knots.
I would say that good performance curves and understanding your boat are important. I feel like our engines tell us, provide us feedback on good fast cruises just as they do on good slow cruises.
For instance, I look at one chart and I notice a 32% drop off in efficiency (nm/g) between 7% and 13% load and then another 25% drop off in efficiency between 13% and 19% load and then another 32% drop off between 19% and 36% load. So overall from 7% load to 36% load you increase speed 89% and fuel usage 440%. This is boat 2 from above. Why such an odd curve? Very heavy semi-planing boat. From 7% to 36% load you're just pushing a displacement boat faster through the water. Speeds go from 9 knots to 17 knots, but at 17 knots you plane and from that point upward then you remain on plane and don't lose efficiency as rapidly. We run at planing speeds, but there are owners who have crews relocate the boats at the slowest speeds, below displacement speed. We run at 17 to 22 knots, others run at 9 to 12 knots.
Now, I do not believe anyone here is going to destroy their engines running them at low loads. However, I do believe that by occasionally opening up and achieving temperatures and reaching 60-80% loads, you will improve and maintain the performance under load. If you never intend to run at even moderate load, may be completely unimportant to you. If you do intend to run at greater speeds, then I think the 15 or 30 minutes every 4 or 6 hours is beneficial. Our goal is to maintain peak performance. If you always run at low load, then that's likely not your goal.