Is a dive knife really that important for divers? If so, why?

Divers carry various tools while diving, each serving an important use.  Some dive experts feel a dive knife is more for appearance and that unless you plan to dive in an environment with serious kelp or other entanglement hazards it should be eliminated from gear.  However, other experts believe you can never be too safe.  A good knife should be included for cutting away or prying items, and overall, for making every dive safer for you, your dive buddy, and the marine life around you.