The only time I've needed a knife, I had one with me but couldn't get to it and wouldn't have known what to cut if I could have reached it (down on my calf). My first stage was completely entangled with monofilament on a night dive in a cold mountain lake. Fortunately for me, a divemaster (not my divemaster) happened across me and cut me loose.
I went to carrying the snips on my bcd. I've been through about 3 sets of them, never used any of them, but they would eventually rust out at the hinge. Now I just don't bother. That said, same as anything of that nature . . . "you don't need insurance, until you need it and then you need it bad".
Lots of our dive equipment is redundant and lots of our dive practices are not necessarily designed to save ourselves, but to be able to save our buddy (or another diver). Maybe I will rethink carrying my snips and put a pair back on my bcd.
A fish and a bird can fall in love, but where will they build their nest?