Hi mrsmitty13,
I wish I could have met you at MnG, but I had to call in sick this year.
To answer your questions,
There are literally endless threads on using (or not) using Spare Air's or Pony's here on ScubaBoard.
In My Opinion:
If you are planning on Exceeding 130ft or doing Planned Decompression Stops, then a Pony is the wrong choice.
That qualifies (to most) as Technical Diving and you need the training and equipment (doubles etc.) to do Technical Diving safely.
The Pony tank in 19/30/40 cu-ft size is useful to the non-technical / recreational diver in Equipment Failure or Out-of-Air situations at depths between 80*-130 feet.
(* The 80ft depth is "My Personal comfort level". Others may want to set it shallower.)
The Pony's purpose is to allow me to do an Emergency Ascent and MAYBE a Safety Stop. Nothing more than that.
The assumption is that I have kept myself in a No-Deco-Stop status and can go directly to the surface if I have to.
You should always plan on completing your dive with the gas in your main tank.
The Pony is a Backup to my Backup (my dive buddy).
It also fits in with my personal belief in being a self-sufficient/self-rescue diver.
I prefer to sling my 19cu pony under my left arm now. I tried a tank-band attachment, but it was not as convenient as slinging it. With the 19cu size, it does not affect my side-to-side trim either.
How did I decide to start using a Pony?
My Experience: recreational diver for over 35 years. I purposefully limit myself to 130ft, no-planned-deco dives.
A previous dive buddy (who has since moved) and I started doing more dives in the 80-130 ft range. Both diving with 80 cuft tanks. Over time, our air consumption started being pretty well matched. We decided that if we were in the last half of a deep dive and had to share air for some reason, our buddy would also be getting low on air by then. Setting up a condition where we could BOTH be out of air while ascending/sharing-air. Not Good. Adding Pony tanks seemed to be the simplest solution.
Given unlimited funds, the Technical Diving route would have been taken. Being "Married with Children" that was also an unlikely occurrence.