UK resident here, in case you couldn't tell :).
Having both in use simultaneously like we have in Canada, where we work with lots of things manufactured in the US or in Canada for export to the US.
So any technician will have to have both metric and imperial tools, occasionally things of very similar size will get interchanged accidentally - using a 3/4" socket on a 19mm bolt for instance, which will work for a while but eventually round off the head because 3/4" is slightly more than 19mm.
(Look up what happened to the Mars climate orbiter)
Probably a similar number of Americans know the same facts. And any of them would answer "How much energy does it take to boil a room-temperature gallon of water" in the only sensible way -- look up the conversion of gallons to liters, do the calculation in metric, and convert back from calories to whatever unit you want (BTUs, I guess).
The point is: nobody, including Americans, uses the customary system in chemistry labs. Nobody ever has cause to calculate how many BTUs it takes to boil a gallon of water without reference to the metric system. So the argument is a bit specious.
This is not the case. This part: "one milliliter of water occupies one cubic centimeter, weighs one gram" is known by most people, even kids in primary school. In other words everyone knows that a litre of water weighs one kilogram, and that there are 1000 litres in a square metre. I concede, though, that at least in Italy, which is the country where I was born and raised, most people wouldn't know the next part: "and requires one calorie of energy to heat up by one degree centigrade".
;)
We are taught to use the scales for measuring ingredients - and thanks to this trick you can weigh water (and milk) and vegetable oil (using 5-10% less) if you don't have a measuring jug.
They taught us that in 4th grade in cooking classes. Then they made us remember that in physics and chemistry classes 2 years later.
Of course I don't remember a lot of things from the 4th grade. This one stands out.
I wouldn’t be so certain about that. I’ve used that to conceptualize volumes and weights ever since I first learned that in school.
"In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade -- which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it. Whereas in the American system, the answer to 'How much energy does it take to boil a room-temperature gallon of water?' is 'Go fuck yourself', because you can't directly relate any of those quantities."
From: "Wild Thing" by [Josh Bazell](https://en.wikipedia.org/wiki/Josh_Bazell).