Once you get a pony you'll understand. You don't take a pony to the dive shop for a fill. You plug it into whatever tank you're diving with a transfill whip, and therefore it gets whatever your main tank has. It doesn't have to be 02 cleaned in order to contain nitrox that's already mixed. 02 cleaning is really for doing partial pressure blending to make nitrox where you introduce 100% oxygen, then thin it out with air. In fact, I don't think pony bottles regularly get VIPped. You could probably even skip hydro if you wanted (I won't be doing that). Some dive shops who sell banked nitrox might insist on 02 cleaning, but that's just a money grab and has nothing to do with safety.
For the most part, my pony gets filled before the first dive of a trip. It only gets emptied if I need to get onto a plane, as you shouldn't actually be breathing from it unless you've screwed the pooch in a major way. However, I have used my pony to work on the bottom of my swimming pool since I only needed 5 minutes to screw in a new intake cover.
Since it's filled with whatever in your main tank, the specific contents require no extra consideration. Besides, even if it had 10% CO it's only for emergencies. I'd prefer 10% co to straight up drowning. At least you MIGHT live.