I really hate to be nit picky (not really!)
I see people talking about 2 entirely different products in this thread with no differention (sp!) There is the dairy product called butter, then there is the non dairy product officially called Margrine, though many, from habit call it butter.
My gut feeling is that Butter may be better refrigerated, though it is not clear that even that is ture.
Magarine on the other hand need not be. I remember having to refridgerate margarine in the summer 'cus it got to hot and liquified if we left it in the cupboard.
The fact is if you are actively consuming these products they should be able to be left out as a cube of "butter" may not last that long. Then again if you are going through cubes of butter that fast you may want to reconsider your eating habits.