Auto throttle must not be disconnected on the 777, but as it's a Boeing you can override it without disconnecting it. Monkey see, monkey do, so the Airbus managers decided you can't fly an Airbus without autothrust, perhaps not realising that the thrust levers don't move on an Airbus so it's not the same as a 777. On the 747 and 737 it's accepted practice to use manual thrust for a manual landing otherwise the pitch power couple destabilises you. Manual flying is approved, all the way from top of descent if you like, just so long as the weather is appropriate and your colleague feels suitably up to speed to operate. Nothing worse than someone saying 'mind if I fly it' as they take the automatics out then promptly proceed to make a Horlicks of the whole thing necessitating continuous prompting. On the 'bus people don't fly manually often because it's been done so many times before and you've had too many sectors that day. On the 747 people don't do it because you're either at a field with dodgy ATC or you are just plain knackered and can't be bothered.