Another non-american here. That doesn't help much.
How often do you try to describe something a sixth of the length of your foot? I'm trying to think of an example where it matters either way, but the only thing I can come up with is that there is only one measurement in metric distance: the meter. A centimeter is a hundredth of that, a milli- is a thousandth. In the American system I have to know the relation between inches, feet, miles and probably more (isn't yard also part of it? I don't even know).
2 inches is a very useful measure for a lot of things. I know that the middle digit on my little finger is an inch, and my foot is actually a foot long. I have no idea what 5mm looks like unless I convert it.
Also, don't forget that wonderful measure of area, the acre. I have no idea how big a hectare is, only that it's less than an acre. And there are 640 acres to a square mile, which is convenient if you grew up in farm country where there is a road pretty much every mile.
I won't bore you with gallons, chains, rods, furlongs, or hundredweights (American or Imperial).
2 inches is 5 cm, very practical too. My thumb is 1 cm wide, the span of my thumb and index is 15 cm. I can add all this up and end upbwith 137 cm, which is a meter and 37 centimeters. Never had to convert anything.
> a "2x4" board historically started out as a green, rough board actually 2 by 4 inches (51 mm × 102 mm). After drying and planing, it would be smaller, by a nonstandard amount. Today, a "2x4" board starts out as something smaller than 2 inches by 4 inches and not specified by standards, and after drying and planing is reliably 1½ by 3½ inches (38 mm × 89 mm).
How often do you try to describe something a sixth of the length of your foot? I'm trying to think of an example where it matters either way, but the only thing I can come up with is that there is only one measurement in metric distance: the meter. A centimeter is a hundredth of that, a milli- is a thousandth. In the American system I have to know the relation between inches, feet, miles and probably more (isn't yard also part of it? I don't even know).