Good Idea I was thinking about the water in the pipe before you wrote this,,This is also a problem in spring, summer, fall. If you have that elbow on there and the pop up that is a large amount of water still in the pipe. Thats a great breading ground for mosquito's. So draining the water out, not only helps with your freezing issue but will also help with the mosquito control. that amount of water can breed hundreds if not thousands of mosquito's a week.