That yuckky "yellow" rind/oxidised bit you get on the outside of the butter is probably for sure, more harmful than the "fresh" butter further inside the bar, so leaving out your butter is a non-starter as it gets this nice rancid rind.

Eww! Mine never gets to that state! At most, the butter stays out for a week. Do some of you not use AC? My house is always 66-71 degrees. That seems to be fine for butter (no, not you, Butter ).