I did not know that there were check valves that make sure that the hot and cold water lines don't mix. Also a mixing valve seems like a great way to make sure that when I am showering and someone is using the bathroom that the water doesn't get hotter or colder. I believe that paying a small amount for something that can help to prevent a bigger problem is always a great thing to do and is always worth it. http://monumentalsupply.com/supplies/valves