Sure, but what should happen and what actually happens, are sometimes two different things. I think the jist of this post is "What do you do when something that shouldn't happen, actually happens?"
Here is my answer to that question: If you go diving with a buddy because the buddy system was taught and understood as a valuable safety element, you get separated and you both make it back to land unharmed, you should have a discussion about that preventable failure and FIX IT.
What do you do if you "run out of air"? Are going to approach the next dive with even more sloppy gas planning? Most likely not.
Would you buy an octopus that, knowing from discussions on Scubaboard, works maybe 20% of the time? Most likely not.
What irks me is that instructors who teach the buddy system, encourage it, even demand it when their liability is on the line, accept -here in this thread- that in reality nobody gives a hoot about actually delivering on the promise. That it has become amusing when a divemaster actually takes his responsibility seriously and gets worried about a "lost sheep".
I know that stuff happens but I do not accept buddy separation as the normal course of business because I do not want to be in the shoes of the father who goes diving with his son at Dutch Springs, looses his buddy, does not find him, and then gets informed at the "lost and found buddy counter" that the body of a teenage, male diver was recovered, who (as later determined) ran out of air, panicked, and died from AGE/pulmonary barotrauma after a Poseidon Missile ascent.
The son's future - gone. His friends and family - left with a void that cannot be filled by a cheesy memorial with photos and flowers. The father - for the rest of his life hounded by pain, guilt, and the question: What if I had ...
The next time you loose your buddy, thank God when you found him/her well by luck or by your preferred method and then FIX THE PROBLEM.
Or, like MaxBottomTime and others, be clear upfront that everybody is on their own and back that up with the required skills.