It's also 16.7 Hz precisely (save for the fluctuations) and not 16 2/3 Hz, for some electrical reasons regarding transorming 50 Hz to 16.7 Hz mechanically, which I don't completely understand
Nowadays with switched power supplies, this is not a problem any more. Keeping track of 16.7 Hz seems a little easier. Imagine building a numeric display for a power plant operator to see how far off you are.
You could build a display with 3 as the denominator, and a decimal numerator:
| |_ 2.1
| | \ ---
| \_/ 3