A pony is a bit like the emergency brake on your car, or a spare tire. You'll probably carry it around forever and never use it, but if you do... just once... you'll think it was the best few hundred bucks you ever spent.
Like Dr. Bill, I dive alone almost always. Even when I'm in the water with friends, we are all essentially diving alone, because we're spread around a wreck and taking pictures and generally being really bad buddies.
For years I carried a pony and really only used it once... when the reg on my main tank froze open at 170'. That little 13' pony allowed me to come off my main tank so I could shut the valve down as I headed up. I sucked that little tank dry in no time, but in that few minutes, my primary reg thawed out since I wasn't using it, and I completed my ascent safely. (Ya, I know, I shouldn't have been at 170', alone, on a single, but I was younger and stupider than I am now.)
Not long after that, I started diving doubles, and that's all I own now.
Like I always said to my students: "Experience is what you gain immediately after you needed it..." Same with a pony. You'll buy one right after the time you really could have used it...
I have never understood everyone doesn't dive with some sort of backup system, like a pony. We're underwater and we don't have gills. This isn't rocket science.