C'mon, these things are not knives. They're last ditch weapons that either fulfill a need or not. They're light weight and strong; they can be stashed on the inside of safes or hung on strings behind doors. They can hang around your neck with no worry about corrosion or weight. And they can be left in the trunks of cars. (There are a number of cases involving abducted women being forced into the trunks of their own cars and driven to isolated places where, when the trunks were opened, the victims put four or five bullets into their assailants. The moral is that anyone can be abducted and that a weapon in a trunk can come in mighty handy. Also, if you have to change a car tire at night. For its weight, a Nightshade can exact a great deal of damage, especially if someone's not expecting it.)
Nightshades also can be used as weapons in animal attacks and can be handy if you need to stop at a rest site when you might feel somewhat less than secure. Two of them can actually be better than one steel knife when one is wielded in each hand.
Why not stash some Old Hickory knives all over the place? About a third the price, and made of 1095 so you can actually put an edge on it. Hell, you can have one in each hand and have enough money left over to put one in your teeth and tuck a spare in your sock. It might rust, but if it's a last ditch weapon you probably won't mind giving the guy tetanus.