Always wondered how those worked, but never looked into it. A friend had one sitting on her stove for years, but never had butter in it. I think she kept it around for the lovely design on the outside of it.
I was brought up to put butter in the fridge unless baking that day. Then it is taken from fridge, set out to soften and use right away. I can't imagine having water sit out that long touching food that will be used, even if it is airtight.