For those who leave their butter out on the counter

I was encouraged by an experienced home chef friend of mine to do that, but I'm having mixed results. I make my own butter and am pretty diligent about rinsing out all of the buttermilk. I store it in a pottery dish that holds the butter upside down in a water bath to minimize contact with the air. I usually add a pinch of sea salt. Room temperature of around 70 now that winter has set in.

Sometimes the butter lasts for up to ten days, other times it is smelling sour after only 3-4 days. What does anyone else get?