(1) Freezers vary dramatically -- a properly calibrated freezer should be below 0F. Many people have freezers that are warmer than this. Our freezer broke at one point, and our landlord was surprised to learn that freezers should be much below 0C (!)

(2) Butter can pick up funny smells in the freezer if it is not wrapped in strong plastic. This isn't technically rancidity, but it isn't pleasant. This depends on what else is stored in the freezer with it, of course, and how long it is stored.