Whether the marlinespike is on the knife or not it is certainly essential gear. Those fortunate enough to have never had to attempt to remove a heavily loaded knot should consider that, without a spike, your only option is to cut it out. If some lubber has tied that knot in the middle of your line you may be able to put a monetary value on the possession of a marlinspike.
A rather nice one can be made, suitable for virtually all size line, from a piece of half inch round stock, shaped on a bench grinder. Make it about 8" long, drill a hole in the end, place a lanyard on it, and hang it where you will. You will find untold uses for it, not the least, as a drift pin.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.