Well I can't speak for anyone else, but I installed mine so I could move the truck off the overload springs. That made for a very rough ride. I've weighed what I tow and am not over weight, close though. Which is why the truck sits down on the overload springs.
I'm sure that some add them because they are overweight, but I think it's mostly to even up the truck so the headlights aren't pointed too high and blind oncoming traffic.
I can't think of any reason to avoid them. If you don't need them they are an extra unneeded expense. My previous 5er didn't weigh as much so I didn't install them until I pulled my present toyhauler.