jlayman800:That's funny!!!! Actually, I don't ever remember using my knife on rope or line under water. I have used it a time or two for prying a shellfish off a rock but never to get untangled. Maybe I will start leaving my two knives on the boat, too. It would sure save some money. I have lost 4 or 5 dive knives in the past 20 years; some of them very expensive.
Depends on where and what you dive, I suppose. Diving wrecks in areas where there's active fishing is almost guaranteed to give you an opportunity for using your knives -- for detangeling yourself or your buddy. Both have happened to me, unfortunately. I've also once had a fisherman drop his net on me (!) while I was on a wreck -- I guess that the diveboat with the flags right above me weren't obvious to him. Cutting-tools came in very handy then, or else I'd have been feeding the critters by now. We occationally run "cutting away nets and lines which may be a hazard later" dives, trying to clean up some of the mess left down there, but it's a loosing battle.
OTOH, if most of your dives are in marine-parks, where you're nice and undisturbed by fishing etc., then a knife is probably not very usefull.