Just try calculating a rafter angle with it some time!
The other difficulty with metrics is that feet and inches, being a sort of base 12, are easily divided in multiple ways. And the half, quarter, eighth, sixteenth, 32nd, 64th progression is logical and easy to carry in your head. The decimals are too unwieldy for practical measurements.
- just a "rough carpenter" - as my dad says, "Good on rough work, and rough on good work!"
Ask a three-year-old to mark the middle of a line, and they will get it right to within 5% or so. Ask a thirty-year-old to mark 0.3 of a line, and he will be off by 10% or more.
The human mind naturally divides things into halves, quarters, eighths, etc. It also divides things into thirds fairly well. Thus the Base 12 system, not surprisingly, makes a lot of sense. I say “not surprisingly” because organically-derived systems often make a lot of sense.
Our numbering system is Base 10 because we happen, by evolutionary accident, to have five fingers on each hand. But 10 is a very unweildy number, factoring into 2 and the prime 5. Splitting it up quickly yields either indecipherable strings of digits, or meaningless decimal fractions. It just doesn’t match the way the human mind works.
Carpentry has its own set of measures, at least for lumber dimensions. It would seem logical that if you stack two 2x4s you'd have a 4x4. No.
Of course, my carpentry experience is from working on old (for these purposes, 40 years and up) houses, where nothing is square, nothing is plumb, nothing is level, and all figures to the right-hand side of the decimal are not to be trusted.