If we're going to have networked devices, we need a foolproof way of disconnecting them. I don't want to have to log in to my pencil sharpener's web management interface to ask it to stop spinning because some teenager in Andorra figured out how to make it spin all night.
Samsung recently got in hot water with their smart refrigerator. Because it failed to validate SSL certificates, the fridge would leak your Gmail credentials (used by its little calendar) to anyone who asked it. All I wanted was some ice, and instead my email got hacked.
We need an Internet off switch.