WsT
Firstly, your instructor is trying to teach you how to manage the descent on your own. It is very seldom that you will fly an entirely managed descent in todays ATC environment. Vectors are a very important tool in the ATCs armoury and then you will need to know how the aircraft is performing wrt the descent. Take heed of his lessons, he is doing you a favour.
Secondly, despite some peoples reverence for the FMGS, it is not the most efficient way to manage a descent, especially in an A320 with IAE engines. It is very conservative(for good reason) and you will often find yourself low and having to intervene with V/S. A point to remember is that while V/S can be helpful, you lose a lot of protections and therefore it is not the best way to descend for extended periods of time.
As to being high, remember your Aero-d lectures/lessons. The co-eff of drag, like that of lift is depend on the speed of the a/c. Therefore, the higher the speed, the more the drag and the quicker you will descend resulting in a requirement for less distance for the high speed descent.
Finally, and for me personally the most important, is that it is more fun to manage your own descent and therefore the most fulfilling. Not much(from a work point of view) beats the feeling of idle from TOD to stable at 1000'(which I have to admit, doesn't happen all that often but I do try).