Maybe I've been doing it wrong all these years. I doublehaul on any and all rods, from 3 wt up. (I'd do it with even lighter rods if I had them.) The intent of the doublehaul is to increase line speed. It is CONTROL of the line which make fly fishing successful. In speeding up the line itself you can reach further yes, but also successfully cast into that 35 mph wind. And something to note here, the doublehaul uses both hands, so you cut down the effort required to cast. You don't get tired as fast and when you're old like me, or fighting a bum shoulder, you need all the help you can get. Once you learn it and practice so it becomes the normal natural thing to do your catching will improve because you will be getting the fly to where you want it.
There is a good video of Castwell doing the doublehaul in the Chat Room...take a look, it's free.