The discussion here is really surprising. We have instructors claiming that it is a situation that we should never, ever have to face and ostensibly can therefore be considered so unlikely as to be unimportant.
The fact of the matter is that running out of air is probably a pretty big factor in scuba deaths. Somebody else can look up the stats, but if it commonly kills people, we should give it very careful consideration. It killed a moderator instructor on this board and a lady I worked with a few years ago, apparently.
So yeah, it is stupid and irresponsible to run out of air, but people also sometimes completely screw up and run right through a stop sign - simply because they were very distracted. They are not TRYING to be stupid or irresponsible, but people (myself included) screw up simple things all the time.
I have run my tank down to very low pressure, many dozens of times. Sometimes intentional, sometimes not. Running out of air at depth (assuming no mechanical problems) does not happen instantaneously. If you scew up and suck on a 6 cu-ft bottle.. yeah.. it will be MT with about zero warning. A full size tank... not so much.
I dive solo often. I carry a redundant system. I have no need or obligation to save air in my main tank for anyone else but me. Sometimes, if I really want to stay down, I will take my tank way down in pressure. When I feel my spg hose go limp, I know I am down too low and need to go up. Sometimes, I might not even look at my gauge.. We know the readings are notoriously inaccurate at low pressures, so if it reads 250 or 150 or less, it really doesn't mean a whole lot to me except that I need to go up.
So what do I do? When the hose is limp...Suck hard! Suck really hard and fast... This will give me a very good indication of how low I am on air. If i feel a slight restriction at the END of the inhalation, then I am not too worried if (say depth is 100 ft or less). I pretty much KNOW I can make a normal ascent and do a safety stop (if I am using a large steel tank). However, I can't breath fast and I can't kick and I can't delay and I must relax. If I screw up, I still have a whole nother tank to use.
If I test the pressure and suck as hard as possible and really get a major restriction in breathing... then I have a problem.. probably not enough for me to make a normal ascent.
A very important distinction should be made that being able to sense this restriction in breathing resistance is HIGHLY dependent on what you are doing. For example, if you are quietly sipping your air, ever so slowly, while you, sit still and try to take macro photographs and you want as little noise and bubbles to be made as possible... it is VERY easy to drain the tank down to very close to zero, without any warning. Conversely, if you are swimming hard and sucking air, you are more likely to feel the restriction/increase in breathing effort.
All this stuff should be obvious, but apparently it is not. Also having a really good regulator, will allow you to breath down further without noticing, but the really hard suck test does work pretty well for any regulator I have used.
My primary point is that, if you simply run low on air at depth... you have air and time to ascend.. It will not just stop.. If you are coming up alone and have no redundancy, it is best to use your precious air to auto inflate the BC, get a little buoyant, stop kicking and then press the purge to deliver air. Pressing the purge will provide air, will not require you to suck and feel restricted air flow and you will be more comfortable. And you MUST relax your legs and not kick, if at all possible. In such a situation, I would recommend ascending at 60 feet per minute for the first 30 feet or so, and then try to slow down to 30 feet per minute.
I think this is more useful then.. it will never happen to you - because you are too smart for this to happen.
Oh, BTW.. it is NOTHING like shutting off the tank valve and sucking the air from the regulator hoses.