Actually, the leather-type, tip-down neck sheath I saw a while ago was being carried by a Native American (Chumash I think).
A conventional non-kydex handle-down neck sheath is 'iffy' to me as well.
As much as I like kydex sheaths (when done well) I have found kydex sheaths a relative pain when backpacking. Strapping a fixed blade on my pack is not only irritating to reach, but a pain to unsheath. I've also tried lashing it to my suspension belt, shoulder strap, arm-band, etc....but due to the force often needed to unsheath, these strapping points have not worked very well. So far, the kydex neck sheath has been most convenient.
I've been thinking about the more traditional style of neck sheaths used by some Native American tribes, and how similar leather neck sheaths might be adapted for today's knives. A lot of fixed blades we use now incorporate a integral finger guard of sorts into the blade or just behind the blade, often used as a 'locking' point for kydex sheaths. A neck leather sheath (tip-down orientation) could use this 'locking' point as well in the form of a slit for the guard, and a strap above the guard to secure it in place.