I truly enjoy the topic of pony rigs and having a redundant air source readily available. There is nothing more important when diving then the safety of yourself and you buddies. My opinion on the subject is that having that completely redundant air source available should be a must for any serious divers especially when deep diving or diving with an in-experienced diver, you never know when having that extra air will be necessary. The big concern is for those who have them and choose to pick what dives to use them on and which not to. The point is having it with you at all times, one cannot pick the dive where the emergency will take place so getting tierd of lugging it around and wearing it on the surface is something you must learn to get past and always have it ready for that instance when you will need it the most. Another reason I am for having that second source of air is because who knows where your buddy will be when an amergency happens. Yes we always are trained to stay with-in arms reach of your buddy but how many times have you found yourself further than that from your buddy in all honesty? We know that right way of doing things but that does not always turn out to be what happens under water. I read an excellent article on the buddy system and solo diving. In this article this diver discusses that when diving with a buddy your safety is oly as good on your buddies compedence. If your buddy is always right next to you and is into pre-dive planning then chances are you will hav no problem but when you dive wityh different people they all say they will be cool in a bad situation but that does not always turn out to be true, so having that extra air source for yourself is also benificial because you then you just have to worry about you being able to reach down and grab it. Buddy diving (as much I am totally for it because I know my buddies very well) is only as good as the level compedcne of the buddy you are diving with.