People secure their Wi-Fi for more than just their connection. There is likely to be file servers, media centres, printers, scanners, radios, TVs, tablets, phones, computers with shared folders, security cameras etc all connected to the local network.

They should be protected by additional means anyway.

Disregarding the fact that this would completely bypass the border routers.

Devices in an internal network maybe protected, but they are never as protected as they are from requests coming from the internet.

Agreed, Google already have all our info anyway so not sure why they would send someone round to connect up to our Wireless networks..