The exact reason for these tests are not difficult to understand. Before the advent of the buoyancy compensator, the diver would descend, become negatively buoyant with depth and have a need to kick like hell to get to the surface. They simply didn't have the option of pushing a button... Safety was dependent on the individual's in-water ability and fitness. The flotation vest was reserved for surface emergencies. The diver used his basic equipment (FMS) to propel himself from and back to the entry point. In an emergency, he had to depend upon his Buddy's ability to perform an assist/rescue on the surface or underwater. Training included rescue skills. The Diver had to be competent.
Since this time, the training requirements have been reduced as a result of the technology that's available. There is a dependence on equipment and its reliability. One thing that I do know is that in the history of man, everything man made has failed. It's simply a matter of time...
It comes down to one's training philosophy and the interpretation of degree of self-reliance that's reasonable for the diver to possess. It's my belief that it can largely depend upon the diving environment. The skill-sets required to dive in warm shallow water in otherwise idealistic conditions, is not the same as ice cold water, surf, waves, currents and poor visibility. The 'Standard' should reflect the diving conditions (which it not always does).
I think that not enough is made of a person's degree of in-water comfort/confidence. The 'comfort envelope' controls panic. Regardless of the situation, if you're comfortable you have the opportunity to deal with it. This is why training programs in the past included blackout drills, station breathing, harassment, etc. (mine still do) and why some instructors still teach Buddy Breathing. For me, it has less to do with the requirement to share one second stage than it does the increased level of communication, co-ordinated movement and increased confidence gained by the Students. Comfort in the water has a very real value. Putting a non-swimmer into deep water is just asking for trouble imo. Like I said, equipment can fail...