As a strictly rec diver in the PNW, I added at 19cf back-mounted pony (119cf HP main tank) as soon as I got comfortable enough with my gear to deal with more hoses. It has a little peanut gauge so I can check pressure when I turn it on but not underwater; it is open throughout the dive but I have the Venturi on its 2nd stage turned all the way down and it has never free-flowed. I had to move 1-2 lbs. of lead in my weight harness to eliminate its tendency to make me rotate. I was pleasantly surprised that the added weight of the tank and regs didn't make it much worse wading into and out of the water.
Initially, I had removed my octo since I had the pony 2nd stage on a necklace, but I added it back after thinking about OOA situations where I might be sharing air with a panicked buddy. There will almost always be a lot more air in my main tank, so maintaining access for me seemed a good idea.
I really don't understand not diving with a redundant air supply - not a big hassle, however configured, and it addresses the thing generally most likely to kill you underwater.