Well, presumably the swapping company would make sure the batteries are well maintained. If someone swaps out a damaged battery, the process should detect that and charge them extra, and not put the bad battery in the queue for swapping.
I think perfecting the whole supercharging system makes more sense.
Factor in the price of recycling with a cynical view of the average user and everyone gets a price decrease on the batteries without the sticker shock of seeing the battery cost all at once. Really you'd sell your Tesla's with an already recycled battery and a couple of years of swapping thrown in. You can price it just like you price car insurance, based on amount of miles driven a year.
Anything that puts more control of the most unproven tech into the hands of Tesla, the faster their batteries will improve. But maybe this is all just too expensive to implement without millions of cars on the road.
The capacity of batteries in rotation would be maintained at a certain level, and the old batteries recycled or used as storage for the superchargers. I think it could potentially cost less than the monthly cost of gasoline for most people.
Superchargers don't use batteries. SolarCity does, though.
Musk said that the ones with solar panels can go completely off the grid and continue to work in a zombie apocalypse. The only way that is accomplished is through energy storage.