Why do we use queen excluders?  To keep the queen out of the honey suppers in order to keep brood out of the honey frames.

When do we put the queen excluder on?  When we add the honey supers.

Does this mean anytime we have any honey supers on we keep the queen excluder on?  What about if we leave one medium supper on the hive for overwintering? Should we leave the queen excluder on?  What is going to happen in the winter as he cluster of bees move up the brood boxes and reach the queen excluder and the medium supper?  The cluster will keep moving up, but the queen will not be able to get through the excluder.  The worker bees won’t know this and they will keep moving up and the queen will be stranded.  With no other bees around her, or few bees around her she is going to freeze and die.  

The WHY we use queen excluders is to keep the queen confined in a certain area.  We use queen excluders WHEN we want to keep the queen from moving up into the upper boxes.  If you forget the why or the when and leave the excluder on in the winter you just created a situation that will kill the queen and therefore the whole hive.  

The WHERE with queen excluders is; maybe down south you can leave them on all year.  Maybe it never gets cold enough that the bees cluster and have to move into the upper boxes in Texas or Florida or California.