A trilobite won't cut rope and depending on its size, shears may not work either. It depends on what you run into, or are doing, whether you need a knife and what size it needs to be. A large knife has gotten me quite a number of rock scallops over time. Came in handy as a hammer a number of times as well.
I have carried my Healthways 7" blade BFK since early in my diving so I'm going to keep on carrying. If I left it home I might wind up swimming in circles due to the loss of weight on one calf. Now I also always carry a trilobite and shears. Occasionally I also carry a small very sharp knife. After that, don't ask, don't tell.
Take what you feel is appropriate for the dive, and you know how to use effectively.
Bob