I think "neck knives" are the most absurd concept..... Defense point of view. What do you say, "hold on while I dig under my shirt". What if you go to the ground. Then the knife flops under an armpit, or spins all the way around and it's on your back...... What do you do, wear it on the outside so someone else has a chance at grabbing it.....then the matter of retention. The chain or whatever you use, is either too weak and snaps accidently are too strong and possibly someone could use it to strangle your "tactical" @$$. It just seems the MOST unnatural impractical place to carry a knife! Utility Point of view. I see no practical uses of having a knife around your neck.
Imagine working on a vehicle or anything wih moving parts And your "neck knife" gets caught on something. Crawling on the ground on your back. Forget about it. Aside you have to utilize both hands to put it away. After looking like a mall ninja pulling it out.