ArcticDiver:
It is the false sense of security provided by the pony that is the primary fault with its' use. Most of the arguments for a pony contemplate an equipment failure and an immediate ascent to the surface. In fact, an equipment failure is extremely rare and should not be the primary consideration.
What's false about having the gas required to get you safely to the surface?
It could be an equipment failure (blown tank o-ring, stuck SPG, back of your reg blew off (ScubaPro recall)) or a human failure (diver screwed up and wasn't watching SPG). In any of these cases, having a redundant reserve that contains enough air to do a normal slow ascent and safety stop can not possibly be a "
Bad Thing".
An equipment failure may be rare, but so is the chance (per passenger/mile) of having a serious auto accident, however we all still have air bags.
So far you haven't come up with any falid reason why it's bad to carry enough extra gas to save your butt.
While everybody is trained that their buddy should help them out if they're OOA, the real truth is that unless you have a known-good buddy, going to your buddy for air at the end of the dive at any significant depth could easily result in two injuries or deaths.
I get a different boat buddy every day when on vacation, and I can tell you that I trust the contents of my pony a h*** of a lot more than some guy I met 5 minutes ago.
Terry