I find that they last longer than bladder and can be oiled occasionally, further extending their useful life. As an experiment, I once oiled pads that had been in an early R-13 for 40 years and, after softening and adjustment, the instrument played fairly well!
I find that they take a better impression from aging toneholes. Slight warp-age, small nicks, uneven wear, can all be forgiven by a leather pad, which will take a better seat than the harder double skin felt pads in common use in modern horns. This often saves me having to reface a tonehole made slightly uneven from wear or abuse, which is my preference when possible: the Hippocratic Law for clarinets being "First do no harm".
I find that they give closed keys a feeling more consistent with bare fingers on open holes (especially on Albert/ simple system horns) versus hard felt. I am often distracted by the difference in feel between my fingers and hard felt or cork pads; its somewhat uneven.
Finally, I find that the tan kidskin shows less wear and discoloration than white bladder, keeping the horn looking fresh longer.
I hope that players and other techs will consider these pads and their advantages next time they undertake the overhaul of a vintage instrument of quality!