The modern nautical mile is derived from the metric system: it's defined as exactly 1852 metres whereas you can only give an approximate conversion in yards or feet. It was established at an international convention and is included in the SI system, the traditional imperial unit is obsolete.
Having 1.000 different kind of measurements for different crafts and professions might be beneficial for those people themselves, but not for others who are trying to make sense of it. If I want to buy a rope, it would be much preferable to measure it in a unit that is clear and understandable to everyone rather than some arbitrary measurement that's only relevant to salesmen in the 17th century. In a few cases (such as the nautical mile) it might be worth to use a non-typical measurement, but otherwise it's much preferable to use universal, decimal units.
I wonder why I bothered to reply though...PVC seems to ignore my posts.
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