my1ocean:
One really can't argue with redundancy. It's always good to have redundancy, but in some cases, is it necesary?
This is the crux of the issue, followed by how much is adequate.
Given that diving takes place in a non life sustaining environment, anytime a diver approaches the boundaries of his ability to easily execute a controlled escape or exit from this environment, whether in terms of depth, overhead obstruction, entanglement hazards or other considerations affecting exit, risk management - which equates to safety, demands gas system redundancy. Failure to employ such creates an exponential increase in risk. It's important to note this increased risk may still be relatively minor and acceptable for some. Good gear is very reliable these days. Good maintenance, along with everything else that goes with being a good, sound, prudent and skilled diver, diving within reasonable limits, results in an acceptable risk vs. reward equation for many diving without it. This includes those who choose a less than optimum, from a safety point, redundant system.
Both a buddy, as well as equipment, provide redundancy. Both present advantages and disadvantages. And for all the buddy talk, tech divers carry redundancy in the form of equipment, as well as in the form of a buddy for most. The buddy does not preclude or exclude gear redundancy. But we are talking recreational, not technical diving, although the lines are not well defined and blur at the edges.
Some deem equipment redundancy necessary at the deeper rec. depth levels. Others beyond their personal comfortable exit level. Others at all levels in this environment. Others choose it due do to questionable buddies or because they do not want to exclusively rely on a buddy for gas system redundancy. Others choose the buddy option up to their various personal acceptable limits. Others throw caution to the currents. Others practice all of the above.
As to how much is adequate, that's a personal decision for every skilled, informed diver to make. I say that because if one lacks the requisite knowledge one can't expect to make a good acceptable, well informed decision. There are some very good pony threads on this board. And reading many of them will provide a well rounded understanding of the issues, along with a lot of nonsense to keep the thinking juices flowing.
The part about full doubles being always best leaves out the specific defining properties which determine whether this is in fact the case. As a generalization it is false. Not everyone has the same needs, dives the same dives, or will take the same path throughout their diving life. The specific requirements needed to optimize for a 30 ft warm water dive are different than for a 120 ft cold water dive. The same applies with respect to the choice of whether to optimize for any one specific dive or for all of one's dives overall. Different circumstances impose different needs which demand different implementations for optimization.
Best - denotes a relationship. Relationships are relative to their specific properties. Only then, from any one specific perspective, can they be seen as an absolute.
I hope am not confusing too much.