I don't think keeping the lure dry is important. That being said, a lot of trappers cover their traps with a piece of heavy plastic and then put boughs or brush over it. The reason is more to protect the trap from snow and freezing up.
I trap over here in the rain and never cover with anything but boughs and the rain doesn't bother.
If I had a lot of cold weather I would probably be using some plastic for covering.