I don't know about the rallys.

Tesla envisions mobile EV battery swapping machines 9/17/17

In theory it's a good idea, but there are too many safety risks. Any contamination on the terminals/contacts from will instantly start a fire (at those amperages), so each swap is a significant safety risk.

Then there's the technical challenge of reliably securing the battery with a reasonably quick exchange.

Then there's the issue of customers recovering their original battery, and the scammers, and complainers, etc.

Plenty of technical and social obstacles to overcome/manage. I doubt they'll ever commercialize it on any significant scale.