I agree with KC, train for every eventuality because should it happen, you can deal with it and nobody gets hurt. don’t train for it, don’t anticipate and more people get hurt. out of air? well we all plan to end the dive with enough air, and DM’s keep checking everyone including experienced divers, but still stuff happens to delay both good and bad, stuff happens to make the anticipated reserve not nearly enough. So PRACTICE, shared air, buddy breathing, staying with the buddy, and maybe you will never need to actually use the lessons, but should you need the lesson, you know what to do. I am interning for my Dive Master certification, people freak out at simple stuff when all they need to do is stand up in shallow water, so we practice, losing a mask, droping the reg, out of air etc, cause you dont want divers to freak out at depth over something simple and easy to remedy. And of course, if you have ever run out of air, and had to CESA or Share Air, you should get religion before the next dive, if you dont well, then you’re hopeless.