There are a lot of humans that live places where we regularly see negative temperatures. Where I live the temperature range is roughly -40–85ºF.
Choosing the freezing point of water isn't really mystic. It lets you know whether it will rain or snow. It also means that any negative temperature is capable of causing frostbite.
Not really, since it depends upon temperature at the clouds, not at the ground level. Oh, and it also depends on atmosphere pressure, and on time (since a phase change requires latent heat transfer in addition to merely being at the right temperature).
And as baddox mentions, 32 is hardly harder to memorize than 0, especially if you know anything at all about computers.
Choosing the freezing point of water isn't really mystic. It lets you know whether it will rain or snow. It also means that any negative temperature is capable of causing frostbite.