I frequently practice with my pony by draining my backgas to rock bottom, switching to pony, then making my ascent. While it goes against the grain of "Emergency Use Only" I feel that by repeatedly deploying my pony under fairly mundane conditions I will be able to do it better when the situation really calls for it. It also allows me to track my gas consumption on the ascent, ensuring I have enough pony for my needs.
You could also do this by checking the pressure at the beginning and end of your dives.
Very true, depending on how much bailout you are carrying. But let's say that we have a minor deco obligation or, in my case, multiple stops along the way. If I know my remaining gas I can decide whether to make all my stops, skip a few, or do a direct ascent and hope for the best.
You are in the Basic Diver forum, which means threads should not consider things such as decompression obligations when determining what awful things can happen to divers. If we were in the technical forums, and people were asking about bailout gasses, I would agree that a gauge is a good thing. I would also get frustrated if a technical diver used a "pony." 
Once again you are correct in that it does not give you any more gas, but if for some reason a diver is breathing heavier (or lighter) than usual, they can adjust their profile. Also, in the case of deco bottles (not deco trained, so this is simply speculation on my part) if one diver lost their deco bottle then he could possibly share the bottle ala buddy breathing with his teammate, if they knew how much gas was remaining in the tank along with their air consumptions and deco obligation.
Once we are in an emergency, there are no real profile adjustments that need to be made. Breathing heavier than normal? Head to the surface. Breathing lighter than normal? Head to the surface. Whenever you are on a pony bottle, you should be heading to the surface in a straight line at a safe ascent rate. Profile adjustments can be made when on backgas and I don't think any of us would advocate using a paintball gauge on backgas, or diving without a gauge on backgas. Unless that is, the user has a J valve. (actually, I was just kidding, and wanted to make that perfectly clear )
Secondly, we are talking about a pony bottle, not a deco bottle. If we were talking about a deco bottle, then different people have different planning rules. Some would use a gauge and each diver would get half the bottle. Others would have the diver with a bottle breathe until his stops were done, then leave whatever is left for the out of luck diver. Then, a gauge comes in handy, but once again, we are not talking about deco bottles so none of my logic has been tailored for deco bottles.
No real argument for that one, save for choosing your equipment wisely (in my case, an OMS 2" brass and glass SPG on a 6" rubber HP Hose, the same way many tech divers do it on their stages/decos). It's give and take, I trade a possible failure point for knowing my remaining emergency gas.
And in actuality, there are few failure points. Anything that leaks onthe HP side will tend to lose gas much more slowly than a leak on the LP side due to the fact that the HP output on the reg is a tiny little hole. I've gone on dives knowing my HP spool is leaking a tiny bit, I just kept an eye on it and followed gas planning rules and what do you know, I survived 
So yes, personally, I like to know all of my remaining gasses.
LOL, I've had practically no instruction on pony bottles, save for what I have gleaned from SB and other sources. You can do it your way, I'll do it my way, it's your emergency gas source. Do whatever gives you a warm and fuzzy, even if that means giving your bottle a fleece liner to keep it warm and fuzzy
. I'll keep mine a Silver Bullet, please.
My deco bottle is yellow 
All the other technical divers with their Silver Bullets laugh at me...(but I got it cheap on Craigslist...)
Peace,
Greg
