I find it amazing there are people here that think a buddy is better than a pony bottle....
A pony is attached to you, always there, buddies are human, prone to making mistakes, the argument that they should be there is not an assumption one should bet their life on. A redundant tank and reg, attached to you is something you can bet your life on.
SCUBA is the only sport I can think of where people typically participate without 100% reliable redundant life support items. (a buddy is not this) It's madness if you think about it, madness supported by the simple fact that SCUBA equipment, even crappie rentals are incredibly reliable and and fail to safe conditions are the overwhelmingly the norm. Pair that with the fact that most NDL dives can be aborted with an ascent that will not likely leave the diver seriously injured and you have enough reasoning for the industry to suggest that diving with a single life support system is actually a good idea. Don't anyone fool themselves into thinking a buddy is better insurance that actual hardware attached to your body.
I always dive with a redundant system, blue water gets a 13 cu ft pony, its barley noticeable and will work fine for NDL warm water diving emergencies. I can't say that NDL blue water diving absolutely requires a redundant air source, you can, after all, just dart to the surface in a worst case scenario (you'll live). But it sure seems to make sense to have a redundant supply to be able make a proper ascent while being calm and comfortable during that emergency.