According to this article (http://www.reuters.com/article/2013/06/21/us-tesla-swap-idUS...):

    A battery pack swap will cost between $60 and $80,
    about the same as filling up a 15-gallon gas tank,
    Musk said. Drivers who choose to swap must reclaim
    their original battery on their return trip or pay
    the difference in cost for the new pack.

So, I think they're not quite tackling the battery quality problem like you're suggesting. I think your idea makes a lot more sense than coming back to reclaim your battery later, though. I find that very odd.