A/T
By T.S.
Not really sure why you'd ever be flying with the A/T not armed (this is different than A/T off).
The only real reason the A/T can be disarmed is in the event of a malfunctioning autothrottle, it's similar to the flight control computer disconnect.
If you do shut an A/T off then you'll get an EICAS message...the airplane isn't normally supposed to be flown that way.