I remain confused by Honda's insistence on such a narrow range of viscosity
It depends on the model of car. The owner's and service manuals recommend viscosities based on outside temperature. On some Acura/Honda cars, they recommend more than one viscosity, typically (but not always) with the thicker viscosity covering a higher temperature range and the thinner viscosity covering a lower temperature range. On my other Acura, they recommend 5W30 as preferred at all temperatures, and 10W30 for temperatures above 18F; in other words, below 18F, only 5W30 should be used, and above 18F, 5W30 is preferred but 10W30 is also acceptable. On my previous Acura, they recommended 10W30 at temperatures above -2F, and 5W30 at temperatures below 32F; in other words, below -2F, only 5W30 should be used, from -2F to 32F, either 5W30 or 10W30 should be used, and above 32F, only 10W30 should be used.
as well as the recommendation for conventional oil. It's a well-know fact that synthetic oil is superior to conventional (although it could likewise be argued that todays conventional oils are greatly improved over those of ten or twenty years ago), and that adding to viscosity range can only help, especially at higher temperatures, detergency and long-term stability notwithstanding.
I don't know where you got this information. There is no such recommendation for conventional oil, or any prohibition against synthetic oil.
Here's what it says in the owner's manual for my 2004 MDX:
Originally Posted by Acura 2004 MDX Owner's Manual
You may use a synthetic motor oil if it meets the same requirements given for a conventional motor oil: it displays the API Certification Seal and it is the proper weight. You must follow the oil and filter change intervals given in the maintenance schedule.
Do all of you run conventional oil in your MDXs?
Many of us, including myself, use synthetic oil in our MDXs. Again, it's fine according to the Acura owner's manual. (And so is conventional oil.) As you undoubtedly are aware, synthetic oil provides advantages of better resistance to breakdown at high temperatures, and better flow characteristics at low temperatures.