One of the key things to the Pony Regulator neck strap is that it can be used without taking the regulator out of the strap - that is more important that it may seem at first. Imagine that an AGA Full Face Mask blows off. The diver puts the pony in his mouth and attempts to re-don the AGA Full Face Mask. First the pony regulator must be dropped out of the mouth to do that. Now realistically, the diver may not be able to clear the AGA Full Face Mask on one breath. Because the pony regulator is still in the neck strap it is right where it was before - if on the other hand the pony regulator was on a BCD strap - then it would have to be pulled out to use it. When the pony mouthpiece is removed from the mouth and dropped, it is not dangling who knows where. Then when it needs to be re-retrieved, the diver could be in big trouble.
It is not realistic to put the pony mouthpiece back in its BCD holder simultaneously while trying to clear an AGA Full Face Mask. Not going to happen. If a diver needs to rapidly ditch gear, the mouthpiece is simply pulled out of the holder. To pass it off it can be pulled out or the neck strap fastex buckle can be opened. A simple chin tuck will bring the diver's mouth to the mouthpiece (after the strap is properly adjusted) which makes for the least amount of time to access pony air, with least amount of effort. The neck location also helps keep the pony mouthpiece out of the mud and bottom debris, and this keeps it cleaner and more likely to stay in its holder.