The only downside of adopting the metric system is less control over room temperature (based on my experience). Every ºC = ºF * 1.8 so have less control (when using only integers to control temperature as is the case in my experience).
Granted this could be solved easily by using .5 (option in air conditioning and heating controllers but in my experience they don’t) for Celsius. For Fahrenheit this works out to enough control for me. For Celsius in a fair number (lets say 15%) of systems it is a bit uncomfortable.
The specific circumstances add greatly to creating a problem. My guess is those that annoy me swing even further than 1 ºC, they move further in one direction in order to not turn on and off all the time. So maybe that moves to swings of 2 or 3 ºC at the measurement point. But that is another issue, the measurement on home (or hotel) systems is often 1 reader so the variation is often greater in other locations.
Add to that the imprecision of their measures, I don’t have good data, but I am confident that the measurement error is fairly high. I am pretty comfortable at about 25ºC for air conditioning. But in some places I am cold at 27º and others I am warm at 23º. It could be me, but I don’t think so (most of the time – sometimes it is me).
A long time ago I had some imprecise portable temperature gauge and while I wouldn’t stake my life on results based on it, it confirmed my feelings (when I felt it was warmer than the local reading said my device agreed and when I felt it was colder my device agreed). Hardly scientifically valid proof, but it made me more comfortable trusting my opinion on this matter anyway.
My guess is in a unit using ºF you often can be 4 or 5 degrees off (or more) in different locations. For some people that might be ok. But for me that often starts to be uncomfortable. If you convert the issue to that time 1.8 it is noticably worse.
Now in reality I don’t think it expands quite that much. While the manufactures balance the confusion of adding .5 to a Celsius controller and decide not to, I would think they don’t swing 1.8 times as far (in heating or cooling in order to not turn on and off all the time), but it is still let precise than using Fahrenheit integers. I believe (hope) they set their internal dynamics not based only on integers but could say for example turn off .5º past the setting and turn on when the conditions are .5º worse than the setting (so .5º too warm in the case of air conditioning, for example).
It is still lame the USA fails to adopt the metric system, but reducing this problem in the USA is one small benefit of holding off I wonder if 1 in a million, 1 in 10 million… up to 1 in 7.2 billion people (just me, all alone in the world) have my concern for the lack of precision of heating and air conditioners when using the metric system.