Apologies for the long post, but this is actually quite an interesting subject.
The English system's units were put in place by the practitioners of industry who needed them and built around the fact that people naturally prefer to work in integers when possible.
You know, HazelGod, you raise a good point. Customary measures evolve for a reason. Standardised measures are imposed for a diffrent reason. They both have their place. Ideally, we can have one system that provides easy customary use and sensible standardised use.
Your observation about integers really stands at the heart of the discussion.
Science, commerce and the public at large will always try to round things up or down to the most useful integral unit.
Hence, both the litre and the imperial quart get a good run in their respective cultures. They're within a hair's breadth of each other in volume. And in abundant common use.
They hold about two day's worth of liquid for a couple; that is, around four servings of milk, juice, etc...
But what about the smaller two-serving size? Because there's less wiggle room in a smaller pakage, these cartons are a little more generous at 600 ml, rather than half a litre. In the USA, they're not a pint, but rather 20 fluid ounces, or 20/32nds of a quart.
Now, it seems to me that neither number is particularly demanding, but the 600 ml designation makes more sense and requires less mental arithmentic to compare it to its larger cousin.
(Plus, why measure anything other than water in fluid ounces, since that's the only fluid ounce of liquid that actually weighs an ounce? It's like measuring water in kilograms--which one could do if one chose, by the way.)
In the real world, little conforms to a precise integer measure. We will always need to deal in fractions, and the metric system makes dividing those fractions easier.
Further, customary measures become less useful as customs change. A grain was useful in pharmacy when there were fewer, less accurate scales--the pharmacist could measure it by eye. Nowadays, I'd be alarmed if my pharmacist used anything but grams and milligrams.
And oil in barrels? It makes no sense nowadays--the figures turn into telephone numbers. One might as well measure it in megalitres, tankers-full, or better, month's-supply.
And, please, somebody remind me. Why, in the 21st century, do we still use the quaint artefact of a nautical mile? Or do we?
Of course, the standard measures will never fully replace customary measures in the popular parlance. Why should they?
I wouldn't dream of ordering anything other than a
pint in a British pub, which the pub police have decided equals half a litre. (They've given me a sip or two bonus, you'll notice). I ask the waitress for a
cup of coffee, not 200 ml. My car gets decent
mileage; 7.1 litres per 100 kilometres. And even here in highly metricated Germany, I order a Mass of beer rather than a litre.
That said, you'd be surprised how quickly language, culture and common practice adapts. Like Fillitup, I lived through the great metrication in Australia in 1975. Perhaps unlike fillitup, I was in my teens in 1975, so I'd already learned the imperial system. In my life, I switch betwen the two reasonably easily.
In helped that in 1966, the easiest and most useful "metrication" occurred. Australia switched from a base 12 currency (pounds, shillings, pence, soverigns, florins, and whatnot) to a base 10 currency (dollars and cents). To help with the transition, the money was colourful, cheerful and felt nice to use--Australians particularly love the $20 note, which because of its bright orange hue, is known as a Lobster.
It proved so easy, and such a boon, that many actually looked forward to the prospect of metrication proper in 1975.
Of course, people didn't abandon their habits of speech. Many are still short of a quid until payday. People are penny-wise and pound-foolish. Slow traffic inches along. A guy is big if he's ten stone. Directions still contain phrases like "a couple of miles down the road you'll see a sign on your left..." The Chrysler 300, say, didn't turn into a Chrysler 5000--though Australians were well versed cc's as as engine measurement, thanks to their experience with European, Japanese and
British cars.
But where one needs to be precise, everyone began to use metric measures unselfconsciously. (Exactly how fast was I going, officer...120 kilometres and hour? How many grams of baking soda in this recipie? How many litres of duty-free am I allowed?)
It was a godsend to carpenters, builders and home-handymen. 5, 6 and 8 mm screws replaced tortuous measures like 1/8 and 3/16ths in. More sensible, since most tools come from Europe or Japan, in any case. And the size of a 2 x 4 was standardised at 50 mm x 100 mm--you may not realise this, but a so-called 2 x 4 measures substantially less than 2 inches by 4 inches, and it has varied from time to time and place to place. I have heard some Australians refer to this woodcut as a "metric two-by-four", curiously.
Some language adapted. "How many clicks was I doing, officer?" means how many kilometres per hour. "Mils" are used when speaking of ml or mm. Blokes buy a couple of kilos of prawns for the afternoon barbie.
That said, some measures still persist. Nobody can quite get the hang of hectares, for example, no matter how many times they see them used in official documents.
I understand that several Australian studies have shown that there have been considerable economic advantages to using a simpler system. Japan and China are Australia's biggest export markets, so going metric eases trade, too.
So listen up, America. The metric system makes more sense, and it ain't so hard to change. All it takes is an enlightened bureaucracy to work out the details beforehand, a positive attitude to progress, and a little goodwill.
(So I guess it's out of the question.)
That's sad, because the sheer size of the US market sometimes drags the rest of the world along with it.
The USA accounts for little less than one-third of all airline passenger-travel, but causes the world's frequent-flyer infrastructure to be denominated in miles. Even Lufthansa calls its FF program
Miles and More.
I've just bought a Japanese TV here in Germany. It has an 81 cm screen*, or according to the literature, a "32 Zoll"--such a description makes sense of the model number, which refers to the screen size in inches for the US market.
Zoll means "customs" or "tariff"--so the model number refers to its customs classification, rather than anything useful or intelligible to, say, a European, Asian, or Latin American. All because the US is still Japan's largest
single export market--a situation that should hold for at least the next ten minutes til China gets here.
Again, sorry for the long post.
*small digression: measuring screens diagonally doesn't actually tell the consumer how big the TV is in any useful way...like, whether it will fit in the corner. A "customary" measurement useful for those who know that a screen is 4:3, or nowadays, 16:9, but makes little sense to the punter. Same with washing machines. What's a 5 kg/12 lb wash? Do I weigh my clothes to work out how big my washer has to be? Better to measure it in bedsheet equivalent or something. But I digress...