With proper training and attention one should never run out of air, just as a pilot should never run out of fuel before reaching the destination. As a student pilot proper fuel managment was drilled into me, and I leave a healthy margin over the required reserves. Of course this only covers pilot error and not mechanical failure.
I'm just getting back into diving after a long hiatus, so I'm applying the same philosophy to diving that I do to flying. The first is to get as much training as possible. I'm in a bouyancy course right now and will be taking AOW on my return to diving trip in Bonaire over Thanksgiving. After that I plan on Rescue Diver training.
I've read the posts about Spare Air, having originally planned on purchasing one, then changing my mind. This was based on the notion it wasn't enough to surface with from depth. I also read the many posts about dive buddies running out of air and pulling regulators from mouths in panic. This got me to thinking about a different philosophy for Spare Air use. What if you had catastrophic failure of your gas supply and your buddy had his back to you several feet away? The few breaths available in the small bottle should be plenty for a swim to him and a non panic passing of the spare regulator (which ever one you want to pass). OTH, the thing is one more piece of equipment to lug around in an already heavy bag, and one more object to attach to yourself.
Personally I am trying to keep my task loading to a minimum. I learned how important that is during my instrument flight training and want to carry it over to diving. I decided to go with a Cobra computer for a couple of reasons. The first is the instrument scan is very easy, everything is on one gauge face and easily read. No looking on the arm, then on the end of the hose, etc. The second is the air integration. I like the air time remaining calculation as it takes math out of the dive. Sure you can calculate remaining PSI for an average consumption for a range of depths, but you have to assume air usage. The computer will tell me how much time I have left based on current consumption (and a conservative calc at that), which will only get better with assent, thereby giving more margin. I like the idea it is tracking my rate of consumption and the extra information will let me track improvement over time.
