It's a private boat, not a commercial operation. I can refuse to allow any non required gear on my boat as I please. Since this particular diver has shown that he cannot be trusted to surface with gas left in his tank I can choose to not allow him to bring any gear which may tempt him to repeat that. In this very thread there have been several posts from those who say they use their pony for a safety stop. Some said they use it, then go back to their back gas. Some even said they use it to extend their dives. I consider a pony to be an emergency backup only, one that should never be used for anything but an emergency.