Hook knives are primarily used in two types of situations. First responders often carry them to cut seat belts or other fabric in rescue operations. Normally things have stopped moving at that point and the hook knife is much safer for the responder and the victim and since it has a razor blade in it may be faster and better than even a well sharpened knife. The second scenario is when it is carried and used by someone undergoing the emergency, as the souls in the helicopter were. Most sport skydivers carry them (I did although I never had to use one). The joke we used to make at the drop zone was that since nobody had them tethered, we would probably fumble it and watch it fall away as we were falling at high speed tangled in our canopy lines, or worse, someone else's lines. I seriously doubt that with no rehersal of a simulated emergency, where you actually unstow the knife and cut a piece of actual webbing that anyone would have successfully been able to do that in the few seconds they had before the helicopter rolled. Especially since there would probably have been people forced together and some pretty violent jostling going on. You still have to grab your tether and get the knife on it in a position where it would actually work. Easier said than done under good circumstances. My thought is that no type of release device is appropriate for inexperienced passengers in this sort of flight. They are all either too difficult to use or would be open to inadvertant release (such as a three ring release) by people who had not done immersive recurrent training. These flights should not be allowed by the FAA with doors opened unless standard seatbelts are worn. These people died because they wanted photos of their feet dangling and people who should have known better let it happen. Someone else can speak to the whole other issue of webbing or gear that can get caught on fuel shutoff lever or someone stepping on the collective while moving around. It's all right there along with insulation blankets or gear that goes out the door and hits a tail rotor. It should never happen.