> That's why you get ridiculousness as things being 13/64th of an inch
Such fractions are very rarely used, you're more likely to use mils (1/1000 of an inch) at that scale.
> or that there's 63360 inches in a mile.
Likewise, something that will probably never come up in your life. Inches/feet/yards and miles just remain separate things, never mixed.
> With a metre and standard prefixes, you don't need any conversion factors, and you can represent any distance at any scale with a single unit.
There's no intuition for them. Knowing what a meter is does not help with getting a feel for a kilometer. They might as well be as separate as feet and miles at that scale.
> Quick, what's 11/64" + 3/8"?
That one's not even hard, it's just a fraction. 35/64"
> Quick, which weight is bigger: 0.6lbs or 10oz?
Another arbitrary problem that will probably never come up, but to entertain you: since 0.5 lbs is 8oz, adding 1.6oz to that (another tenth of a lbs) results in 9.6oz. 10oz is bigger than 0.6 lbs. Not hard, but at least mildly harder than the first question.
None of this really had to do with the convenience highlighted initially: 12 inches in a foot and 3 feet in a yard make extremely convenient divisible factors. You can trivially divide things by 2, 3, 4, and 6 and keep with whole integer values. The same definitely cannot be said of metric.