When you change the heater out you can avoid draining either hot or cold plumbing systems.Somewhere I saw one plumbed with shut off valves on both hot and cold. What's the benefit of one on the hot side?
I only put the valve on the cold inlet because my water heater is not in a basement. I also put my expansion tank right on top of the water tank with a tee. I installed the cheap expansion tank just in case since I am on a well with 4-80 gallon expansion tanks but what if some fool shuts a valve?