To follow up some of what others said. I mainly use a pony when I solo dive a local quarry. I use a 30 cf. I like to turn the valve on to pressure my reg. on the pony, then turn it back off unless needed. Idea is to prevent water getting in through the 2'nd stage (it's a piston reg.; seems like I read somewhere it's good to keep it pressurized when rinsing, or to avoid having the 1'rst stage lower than the 2'nd; I could be wrong, but it's what I do). So every dive the pressure in my tank drops. Since a fill at our LDS will probably cost the same as a regular tank ($7, or a punch on my air fill card), and I don't have to have the full 30 cf to be comfy with it for the dives & conditions I'm doing, this saves a bit of money.
But I sling mine & don't plan to take it on plane trips. And I'm a big guy with a SAC (around 0.65 cf/min., give or take) that's maybe decent but not great.
As for deco., which I'm not trained in & haven't run into yet, I figure if I accidentally got into it, I'd catch it quickly, have a brief obligation, and 'follow the computer's' advice on what depth to go to & how long to wait.
Having to skip deco. to reach the surface without an OOA situation is conceivable, but should be highly unlikely.
Richard.