I bought a Tusa flat tip knife last year even before I've gone for my first open water dives. On the 2nd dive to get my Open Water certification, I didn't bring it on the dive. Was left in the room. How I wish I'd not bothered about what people thought of a newbie strapping a knife onto his leg. We came across a poor stingray with a hook in its mouth and a small sinker attached to the other end of the line. Obviously the angler had tugged hard and the line had broken off at the sinker. If I had had my knife with me then, I could at least have cut the sinker part off so that the poor ray wouldn't get itself stuck somewhere if the sinker had caught onto some coral. A 'normal' sized knife at that time would have allowed me to cut the line from at least 2 to 3 inches away from the frightened ray so yeah, I carry a big one and so what? It may come in useful some day.
ps. One of the instructors did try to get the hook out but the ray was getting more and more stressed and swam quickly away in the end.