heres a tried out theory(Done in the pool i hasten to add!) A DM said to me when i was doing my rescue course, "where else do you have air if you run out"? threw me, BCD he says! as you ascend the air expands, it might taste like the proverbial but its air, so we tried it, got neutrally buoyant in the pool (3 mtres) and sucked off the BCD! we got about 3 good lung fulls before it gave up. would this be poss do to in a emergency?
You will probably get more than that from your tank by inhaling through your regulator as you ascend, and it will be nice, clean air coming through a device designed to give it to you. Remember, your tank is not out of air. It just can't give it to you at your present depth because it has to have more PSI than your ambient pressure. I hope your experiment didn't put too much unpleasant stuff that has been hanging around inside your BCD into your lungs.
This idea has been discussed quite a few times, and I agree with the majority who say there is no point in it when the alternatives (breathing from your tank and CESA) are readily available and easy to do.