Somewhere I saw one plumbed with shut off valves on both hot and cold. What's the benefit of one on the hot side?
When you change the heater out you can avoid draining either hot or cold plumbing systems.
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?