Fahrenheit's hometown is certainly the metric everyone should use /s

Celsius is not arbitrary, it is based on objective physical reality, and the only arbitrary thing about it is atmospheric pressure, which is more or less equal on the sea level. The rest is us finding convenient patterns, not the other way around. 0-40 is not a scale, it's an arbitrary range and adaptation of Celsius to subjective feelings of hot and cold - one that you ironically need for Fahrenheit, too. Actual thermometers normally go -50°C to +50°C.

On sub-zero, it is the same idea: -5°C is a weather for a light winter jacket, -15°C is a weather for a heavy winter jacket, -25°C is for heavy jacket and some pullover, etc etc.

The 0-40 argument demonstrates that we don't need some arbitrary scale based on Fahrenheit's recording in his hometown in order to conveniently estimate temperature. The groups for each dozen of degrees are just for easy reference. 17 degrees is optional for your taste, to me it's light jacket weather in overcast or t-shirt weather when sunny. There are no perfect temperatures for anything and anyone, and it just doesn't make sense to get into more detail.

As per granularity, people invented decimals, but normally it's simply not necessary to tell the difference between 17°C and 18°C, let alone between 63°F and 64°F. There are so many factors influencing the temperature feeling, and one degree ain't one.