HF
you may be referring to the type that I fly. It is indeed SOP to fully couple at Vy. That is what the aircraft is designed to do and it does it very well.
It is perfectly acceptable, if briefed, to manually fly it all the way. It will do it, just not as well.
What is not acceptable is to 'mix-mode'. Coupling to ROC without IAS is a no-no as, in the event of engine failure you will be at great risk of rapidly getting on the back of the drag curve.
When the aircraft was new, pilots converting from steam driven types would, anytime the AFCS/FD wasn't doing exactly as they wanted, pickle it and go for full manual. Trainers had to emphasise the benefits of being fully coupled and encourage its use. Now we have learned the foibles of the system, we can fine tune it and, if necessary de-couple unrequired modes. Perhaps, with a new generation of pilots who have not had the benefit of thousands of hours of uncoupled flight, we should be changing the emphasis back the other way.