I'm not saying it makes a ton of sense, but my only real defense of it.
That said, I find negative units rather silly in most measurements, so we should really be using K I think.
That's your choice though. Outside temperature cannot be measured precisely enough to warrant decimals (it's supposed to be 17.4°C outside but who can tell the difference with 17°C or 18°C...) but my car's A/C is set to 21.5°C and my home's heating is set to 19.5°C.
Ovens aren't accurate enough for measuring 5°C, let alone 2.5°C.
(Ignoring badly-translated American recipes which call for 179°C and 454.5g of potatoes.)
For air temperature, my behaviour wouldn't change at all if someone told me it is 18±0.25°C rather than the normal 18±0.5°C. We could round to the nearest 2° and it would be fine.
> badly-translated American recipes which call for 179°C and 454.5g of potatoes
This drives me up the wall .. but a lot of UK goods are sold in "metric-numbered imperial sizes". At the weekend I bought 450g (half lb) of meat and a 6.3mm (1/4") router bit.
Measuring body temperature with precision of 0.1°C is very common. In fact, I never saw an integer in this context.