It will be great if US and UK use the metric system like the rest of the world. However, human beings are reluctant to change. We are creatures of habits. If one grew up in an environment where imperial units were used, then metric will appear strange and vice versa. The older a person, the more difficult it is to change. Just getting the windsurfers of the world to use the same unit of measurement is impossible!
The great thing about the Beaufort Scale is that it is independent of the unit of measurement because the same wind force can be converted to m/h, m/s, km/h or knots.
(It is interesting to note that not all things nautical are measured in knots. F1 powerboats are measured in km/h or m/h.)