Source: http://gizmodo.com/stop-refrigerating-yo...

It's likely that you keep butter in the fridge because you think it's safer that way. After all, butter is made from cream, and if you leave cream out in the open for too long, it can go bad. And if you consume bad cream, you could get sick, if not from the rancid taste then certainly from the foreign bacteria that feasted on the stuff when it was out in the open.

This line of thinking doesn't really apply to butter, though, at least not in that extreme. The cream that's used to make your standard market variety butter is almost always pasteurized, and it takes quite some time for pasteurized dairy products to go bad. Foreign bacteria is still attracted to pasteurized dairy products. Butter can indeed go bad. But as long as you keep it covered, it's very unlikely that anything bad will happen to the butter before you've had a chance to eat it all.