I think (just like the thermostat). This is a trojan horse. Nest trying to get a sensors into your home. Specifically presence sensor in every single room of the house. This will be their foundation for a vastly improved home control UI. They will take the lessons they learned with the learning thermostat temperature control and use that for the rest of your home, with the input being location, number of people, etc...

At least that is what I would do if I were Nest :-)