Some years ago I dispatched with (dare I say it? 'just') the autothrottle u/s (A300-300). Everything else on the aircraft was working, including the auto pilot. My FO was an ex-cadet and on the first sector, (DXB-AUH, a distance of only 70 nm), it was his sector.
It would not be an exaggeration to say that for me, it quickly became something approaching a pre-solo instructional ride. (I have done many such rides, if in aircraft considerably less complex than an A300.) He was totally overwhelmed - just by having to move the thrust levers himself whenever he did anything else. For instance, on descent, he could not understand that winding back the rate of descent on the V/S knob was not enough to slow the RoD and maintain airspeed. Just couldn't see it. Nor could he understand that pushing the thrust levers forward would reduce the RoD if he commanded a particular airspeed on the autoflight system.
On the second sector, (mine - AUH-LGW), we talked at length about what I was doing to maintain the flight path using the thrust levers in concert with the autoflight system. I had the distinct feeling that all that I said was met with glazed eyes and the attitude that why should he bother? It was never going to happen to him again.
That flight brought home to me very clearly how quickly the whole operation can go off the rails with just the most simple systems malfunction with a crew who have been trained from the start to use full automated flight without a clear understanding of the basics - and how to operate the basics.
The US Navy maintains that carrier landings involve a highly developed skill set that will be lost unless it is constantly practised, to the point where they stop paying pilots the extra pay loading this skill attracts if they don't maintain their recency. Flying a modern airliner is considerably more simple and less demanding than landing a high performance jet onto an aircraft carrier. However, the same rule applies - if you don't practise the skills you may one day require, you'll lose them.
And if, as a younger pilot, your airline policy prevents you from ever developing those skills (apart from a 'dip your toes in the water' moment when you do it in carefully controlled conditions once every six or twelve months (if that frequently!) in the simulator), how can you be expected to deliver the goods if you require those skills in less than ideal conditions? We've had a few examples of this lately - for example, the one that seems to have confronted the Air France A330 crew out of (was it?) Rio and the turboprop at Buffalo.