If they predict sub-zero temperatures in early spring you have to cover your plants in the garden or they are likely to die. If you leave your house for a week or longer and they predict consistent sub-zero temperatures you have to set heater to keep it above 0C or drain your pipes so they won't burst. That's pretty practical threshold if you ask me.
Nobody says in the weather prognosis "beware people it will get sub-negative 17 C at one point", they just say the lowest it will get. But they do warn if it will get sub-zero C at any point, there's a separate word for it.
> Any time it is above ~0F the roads will de-ice during the day from the salt they put down.
The ~0F for salt-water mix is a very fuzzy threshold, depending on the proportions it can be anything up to 0C in reality. In practice I just expect ice if it's sub-zero C, I don't really know if they will salt the roads before I'm there and how much of it will remain when I'm driving there.
I also don't understand why you need 100 degrees when you can't distinguish between for example 70 and 80 F. Could just as well be using dekoF without perceived loss of precision.
> Powers of ten are just scientific / engineering notation, they aren't really all that dependent on the underlying base units.
The problem is converting between feet, pounds, inches, miles, gallons, etc. It introduces ugly factors everywhere. Sure you can do it, but it turns something you can do instinctively on the fly with metric into something you have to solve on a paper or with a calculator.
Fun exercise to showcase this - how many 50x50 cm tiles to tile a 2m wide pavement 5 km long? Now do the same with 20x20 inches tiles for 5 feet wide pavement 3 miles long :)
See roman numerals vs positional system. Multiplication with roman numerals is a daunting task, while kids in primary school can multipy long numbers with positional system. Despite that there was over a century where both systems coexisted, and some people even complained that "positional system makes falsifying accounting books too easy".
Also 1 kg ~ 1 liter for most liquids is very useful for quick estimation of mass and volume, can't do that with pounds and gallons.