If you’re designing an object (e.g. a 3D printed shell) then you need to know precisely how big a board is to know if it will fit.
“Very small” isn’t precise enough for design work.
Imagine if you tried to build the Arduboy’s shell without knowing precisely how big the guts were going to be,
just roughly how big they are compared to the coin of a foreign currency.
Identifying the numbers on a ruler would take no more time than identifying a coin.
You don’t have to count the notches, every ruler has numbers printed on them.
In the era of online shopping where companies will ship to other countries they ought to be making an effort to consider the needs of those other countries.
If they only target their domestic market then they’re not making the most out of having an online shop.
I have at least three rulers on my desk, plus a set square, plus another ruler and two more set squares in a drawer.
There’s also probably some tape measures in a drawer somewhere.
Go out and buy a ruler, you could spend one of those quarters and still get change.
At the very least you would be able to draw straight lines.
How exactly is that a benefit?
It doesn’t matter if you crop off the far end of the ruler because the larger values are irrelevant.
That’s not exactly hard either, just chuck a piece of A4 on a table.
Having decent lighting is the real challenge.
They are, but not all are standardised or in global or even common use.
Having units that are globally recognised and generally in common use is important.
Not enough to be perceivable by the human eye.
As @Keyboard_Camper said, inches are also standardised and well-recognised,
so no, it’s not worse than using coins.
It’s not as good as centimetres because less countries use inches,
and SI units are the standard for doing anything science-related,
but at least there’s a fixed inch to cm ratio if someone wanted to convert the units.
Also the majority of rulers have both centimetres and inches anyway.
Another reason for not using coins - coins are generally circular, which makes it harder to judge the scale by sight because you end up comparing height/width to diameter.
Using something with a flat edge makes it easier to see the scale because you’re comparing height/width to height/width.