Good aperture sights are a huge improvement over open sights. I wouldn't scope that rifle either. The 45-70 is not a flat shooting caliber, but with good aperture sights, it would probably be decent out to 200 yards or so. I would think that some of the Skinner sights are probably more rugged, but I am partial to tang-mounted sights on lever-action rifles; Marbles and Lyman make some good ones.
Certainly, a scope opens up greater capabilities, but why handicap those advantages with a 45-70 when the scoped rifle stands out with a caliber that can utilize it better; like a 270 winchester.