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xkcd #3001: Temperature Scales
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Converting between Kelvin and Celsius is simple addition; converting between Rankine and Fahrenheit is simple addition. Converting between the two groups requires multiplication, and pre calculator, that's notably harder.
Also, all your kJ/kg/°C or BTU/lb/°F tables and factors are identical when you swap to referencing absolute zero. If you change to the other unit system, all that goes out the window.