I can't count the number of times a buddy has been handy during a dive. Not, perhaps, essential for survival, but handy. Many difficult entries and exits are rendered much easier if you help one another. Getting back on a pitching boat can be much easier if there is someone to haul on a tank valve or manifold. Navigational confusion can sometimes be resolved if two people confer. Minor gear issues can be challenging to handle yourself, but easy for a buddy (she says, remember NWGratefulDiver refastening the cambands on her tank at 100 feet one afternoon). A couple of my narcosis-induced navigational errors in caves have been caught and corrected by my team.
Loss of consciousness, while driving or diving, is a bad thing. Yes, a passenger may be able to get the car to the side of the road, and prevent an accident (although it often doesn't happen that way -- I know, because I see those accidents in the ER). A dive buddy MAY be able to get an unconscious diver to the surface. But I don't plan my dives or my drives around such a rare and unforeseeable event -- if it IS foreseeable, most people would say that such a person should not be either driving OR diving. I plan my dives around the foreseeable problems, which range from getting my light cord tangled up to violent freeflows, and in all the cases I can foresee and plan for, a buddy would either be handy or necessary.