I think they are still put on leverguns because that is a traditional sight. I've read that was the original BDC sight system. For a real long poke you lined up the front sight to the top of the buck horns, for a mid range shot you used the top of the sight notch and for closer you pulled the front sight down deep into the sight notch. Of course you'd have to shoot and figure put what ranges corresponded to these sight pictures. A ladder sight is an easier and not much slower sight to use IMO.
I dont care for buckhorn sights, so most of mine get some sort of tang or reciever mounted peep mated to a blade front sight.
