Cars that idle around town (or boats that idle their engine to charge) do not get to operating temperature in the hot end of the engine. Most of around town operation is at idle or accelerating - the latter I can see as being bad for a diesel with a traditional mechanical injection pump, since excess fuel is injected before the engine comes up to speed.

However once you are operating steady state at some significant output, say 20-30% of rated power, the engine should be hot enough. Sailboat auxiliaries do not suffer from any ills due to acceleration (absent a manic helmsman), the throttle is changed infrequently and there is no inertial load to speak of.

The VWs you mention that were driven on the freeway where not operating at 80% power. The old (lower power) ones, maybe 50%, the newer ones, maybe 20%. Just like my Ford, it is nearly impossible to run a VW TDI at near full throttle for an hour except on the race track.

I accept the "don't idle for extended periods of time" mythology as having a plausible explanation in fact. I question though, the "run it hard to make it last long" mythology as I can think of no plausible explanation, nor have I seen any convincing data to support it. Bearing life and ring life are both related to speed and load. Valve life is related to speed and EGT. All of these go up as the throttle is opened. Without a more than offsetting effect with increasing speed and load, wear will go up and longevity down. From idle to moderate load there may be such an offsetting effect: lubrication is better as oil pressure comes up, proper operating clearances are established at temperature, cylinder walls and intake temperature are sufficient to keep the fuel from condensing and ruining the lubricating oil film. Once that point is reached, what are the further benefits of speed and load?

I am willing to have my mind changed on this, but I am a "show me the data" kind of guy. Ideal would be a controlled experiment in which identical engines were run from new to end of life, one at say 80% load, the other at 30%. But what I have usually been given is a testimonial of the sort, "I always ran mine hard and it lasted awhile..." which tells us essentially nothing.

I should add that I don't think running a diesel at near full load for an hour every month or two does it any great damage, and if it spends the rest of its time at idle it might even have some benefit. It is rather those who say a diesel should always be run at 80% or more of full power to achieve long life that I doubt.