The end ratio of high gear is going to be in the neighborhood of 0.68/1 no matter if it's a 6 speed or a 20 speed. Assuming we're talking HD gassers, which hit their powerband at a high rpm, other than squeezing an almost impossible to notice MPG gain the extra gears don't give you all that much.
Now in a truck that's used by Joe Schmow to haul a couple of bags of mulch between work commutes, or Soccer Sally to haul home the groceries, which are what the vast majority of light duty trucks are used for it makes all the sense in the world to have some extra ranges between top and bottom. In a HD truck being used as an HD truck it does not make as much sense. I suspect I am getting an 8 or 10 speed through no choice of my own the next time I buy a truck, but my current 2015 GMC 2500 with its 6 speed has more than enough intermediate gears, I don't know what I would be gaining with 2 or 4 more other than more heat from more shifts and additional parts to wear or fail internally. I was not looking for mileage when I bought it.