IF you're looking into a truck that already has a double rear axle, then great, it'll help. I don't know about putting one on, unless the truck is somewhat new, tho. But I LOVE having a class 8 with duals. 2 of the last 3 loads I got paid t/t rate to the truck. One was 14k, and the other 17k; so dispatch does know to watch for those kind of loads, if your truck is capable of hauling them.
You don't need them to make money; however, they end up being a nice bonus.