Also, a blanket ban during takeoff (seriously, it doesn't take that long!) avoids arguments with cabin crew about whether or not your device has wifi or not. Are cabin crew really supposed to keep current on all models of new personal electronic devices?
First, the ban is not just "during takeoff". It's actually from the time the pilot leaves the gate, until well into the air. If there's any delay getting on the runway, it's easily 45 minutes or longer, and the usual wait is closer to 20 minutes.
Both ends of the flight. On short flights, you'll spend more times without your smartphone on then you will with it.
