The nautical mile (6076.12 ft) is a good example of a useful occupation-specific unit. Because it was originally made equal to the length of an arcminute of latitude, a navigator could easily read distances off his charts marked with latiude and longitude. However, since the earth is an oblate spheroid, the length of an arcminute of latitude is not constant so the unit value was indeterminate. Given the utility of the unit though, it was incorporated into the SI system and is now defined as exactly 1852 meters. The unit of maritime speed, the knot, is also included in SI and is set at one nm/h (1852 m/h).

The trick with such useful derived units is not to let them loose in the world at large. Use of the nautical mile is largely confined to the areas of nautical and aeronautical navigation. Nobody is putting up road signs showing distances in nm, nor are drapers selling cloth by the milli-nm.

Chalking the number of times a sheet of steel has been through the rollers is probably useful in a rolling mill. But once you start selling sheet steel forget those chalked numbers and list it by thickness using the commonly accepted measurement system (which means metric most everywhere).