e.g. If somebody asks me "How tall is that person?" it's easier to say "About five foot" rather than "I dunno, about 155cm?"
(I'm British, btw)
It isn't, really. If asked about my approximate height, I'd say 'eins-achtzig' ie 1.8m (notice me omitting the unit), which in practice also has only a single significant digit (or 2 if you account for tall people) and to me isn't any more unwieldy than '6 foot'.
I'm totally unable to represent in my head how really tall "about five foot" is whereas 1,50m is totally understandable for me, easy to visualize.
It's confusing, but makes sense in the long run.
You're perceiving things like this because it's what you were brought up with. None of the examples in which you claim it's "natural" to use imperial units make any sense to me at all. I don't think in miles, don't know how much a "stone" is, and pints are just pure confusion - which pint? there's about 4 types isn't there?
It's what you grew up with, nothing more.
The 30cm unit would improve absolutely nothing, it would only complicate things. If you want to use approximate measurement, just round it to tens of centimeters. Or you can use meters with one decimal place, whatever comes more natural.
I totally agree. Rounding to powers of ten works comes for free in metric measurements using base ten numbers. This is by design.
And what if your feet are not 30cm, but longer or shorter?
If you say "about x" you might as well estimate to 10cm precision and say "about 180cm" or "about 190cm".
This all apart from miles of course, but when all your road signs are in miles and the speed limits in mph there’s not much you can do about that. Who knows what plans there are for that switchover…