Let's understand what this is: it is a MagSafe Charger that, when needed, you can take with you to charge on the go. It is not an add-on, semi-permanent, battery.
I believe, and would love to see pictures, that this can be used as your standard overnight charging puck. But it also doubles as a separate battery pack, like the ones you buy at airports, or that come with your luggage and you always forget to charge the night before you have to fly.
This is a Battery Pack that is always charged, if you use it as your main MagSafe Charger, and you just pick it up on the way out.