Yes indeed, but the German-Swiss-Austrian 16.7 Hz system is many orders of magnitude bigger than the Southend Electrification! The path dependency is much more understandable in that case.
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