Ok here is my 0.00001 cent's worth.
Here comes the BIG reason why the autothrottle is ON (normally) during any approach on a 777, as opposed to any older type of boeing.
The tripple is fly by wire. You don't need to trim the airplane, exept when you are changing the airspeed. So configuration changes (flaps) have no influence on your in-trim condition.
Example: config = 180kts flaps 5. You want to configure further for landing. So you ask for gear down flaps 20, and if you want to leave your speed at 180 kts, YOU DO NOT HAVE TO TRIM. All effects such as ballooning due to more flaps, are trimmed away automatically. Also, the autothrottle will help you enormously by adding more thrust due to more drag. This added thrust would normally create a pitch up effect due to the under-wing mounted engines, but.....THIS EFFECT IS ALSO TRIMMED AWAY.
If you turn off the autothrottle, you will have to add thrust youself. All your own inputs for manual thust changes are going to be trimmed away by the auto trim function. So basically YOU are flying the aircraft, and someone else( the autotrim) is trimming!!!
This is cumbersome and sort of more difficult, but not impossible. Maybe not desireable after a long haul flight though.
However.
When you turn off the pmc's this will also turn off the autotrim (and much more such as the autopilot). this is great fun because it will turn your 777 in a large 737!!
We had to do this some time ago in a type recurrent.