Use mine all the time. The E450 Drive train is hauling around a lot of weight with my particular set up. It reduces gear hunting and keeps the transmission and engine speed where it's most efficient. It also helps when descending a grade. Your manual will provide a recommendation for how it's designed to be used. Some of the smaller units would probably be fine not using it but, on the 30 footers, I am thinking that using it would be an advantage.