You can add any kind of divisions in the ruler, not matter what unit you use.
It's just a historical curiosity.
Metric rulers just don't use those because there is tradition of using different fractions. In metric you typically use: 1, 3/4, 6/8, 5/8, 1/2, 3/8,1/4, 1/5, 1/8, 1/10, 1/20, 1/25, 1/50, 1/100...
