Can Butter Go Rancid?
If covered, I wouldn’t be worrying about bacterial spoilage. There isn’t enough available moisture in butter for them to grow fast. However, butter goes rancid quite quickly in warm rooms or when exposed to sunlight. Some people (including me) are more sensitive to the taste of butter just starting to turn than others. I can’t stand it when other people happily chow down. Rancid fats aren’t going to hurt you unless you make a habit of eating them, but they taste yucky. Whether I side with your roommate depends on the temperature of your house.