Why not let diver who NEEDS the air have it without complicating the situation with knives and holdbacks?
Because you are assuming this OOA diver is rational.
From my classes I have learned this:
If you run out of air, you are not a very expereinced diver. You have messed up. Sure there are other situations that may arise.. Equipment failure being the primary.. but.. aside from that, I do not see how any "real" diver should expereinece an OOA scenario.
I am an air hog. I suck air in like it is going out of style and that is at my most relaxed. I just move a lot more air. (Personally I blame it on being a BMX bicycle racer and in that sport we have to breathe a lot!)
I watch my air probably more than anyone else on the trip. When I hit 1k I am winding my dive down.. by the time I hit 800 I am already slowly moving up.. I will always finish with about 300-500. This is regardless of shore diving or going off a boat or river diving.. I watch my air because its the only way I can ensure I am going to be around to raise my kids and keep them diving.
Now, to assume that someone out there, that is ignorant enough to run out of air, is going to remain calm while they come for my air.. No way.
During my AOW training we are taught how to signal without any devices (palm to fist hard makes quite a sound under water) and then to wait for other divers in the area to come to you for assistance. If you come at me, I am not giving you my air.. I will go to you.. If you are not rational and under full control of your possible panic mode, Ill let you float first. I have no issues saying that in any given situation I>U.
There are some very real dangers but the #1 rule I was taught is that each and every dive I do is MY DIVE. Not some DM, my buddies,or anyone elses but mine.. My buddy (my wife) knows I use about a 1/3 more air than she does. She could have more bottom time without me but has graciously accepted that when I am done, we are done. Thats part of that responsiblity concept. You need to watch out for you.