Being a dyed in the wool traditionalist I can't warm up to any of the hard plastic type knife sheaths or Handgun holsters no matter what specific material they are made of, and I'm not much of a fan of nylon sheaths either.
I have to admit to being a knife nut and owning many more than I actually need, many are the newer production knives that came with the hard sheaths or nylon which I usually discard upon opening the box, in my opinion you can't get a better sheath or holster material than good old fashion leather.
Of course to each his own, use what you like, but leather is traditional for a reason, it works, providing one properly treats the leather and keeps it well maintained it should last a lifetime even under hard use.