Every Nest smoke detector has a motion detector, which is then used to feed into the "Auto-Away" aspect of the Nest thermostat. Also, if the carbon-monoxide detector alarms, it tells the thermostat to turn off the gas furnace. That's actually pretty cool. What's interesting to me is the whole house integration (e.g., "smoke detected in the kitchen" being announced throughout the house) as well as the integration with the app (e.g., a push notification to your phone that an alarm has sounded).

Why do I need 7 of them? My house was built in 2007. It's got four bedrooms, each with its own smoke detector. There are two more in various hallways, and one downstairs around the corner from the kitchen. I assume that it was some sort of building code that led the builder to install so many of them. I figure, if I'm going to do it, I might as well do it right and see how well it works.