1st: Temperature swings is going to cause moisture no matter what you store the powder in, closed container or open powder measure.

2nd: Are there any young folks around that can get at the powder more easily when it is in the measure? Even if they have been taught not to touch such things, temptation can be great for a child. No use putting it right before them.

3rd: What keeps you from putting it back? It takes like two seconds to remove the turret and the lid to the measure and dump it back in it's container.

4th: It will discolor the container. Not a big deal but, to some, aesthetics is everything.

All of that being said, I will leave powder in a measure only if I am going to continue loading that cartridge the next day or I am working up a load and will run more of that powder within a short amount of time, no more than a couple of days.

My reloading room can be locked so curious eyes could be kept out. Although all I have left at home is a 17 year old girl that doesn't show any interest in reloading!