Tesla's battery packs are distributed throughout the car, both to allow for cargo space (not having a trunk would be something of a minus) and to distribute the weight of all those batteries. I am having a very hard time envisioning a way to achieve both of those goals while making the batteries replaceable.
It's just that for the majority of people, battery swapping makes no sense. They can charge their vehicle over night at very very cheap prices and have enough range to serve them all day. If you insist on taking a cross-country roadtrip, rent an ICE - the infrastructure is just not there yet (or peoples minds have not accomodated - see this article).
Pic of the battery: (which is really the base of the whole car) http://www.cartype.com/pics/10143/full/tesla_model_s_battery...