It is important to make a distinction. Walker's performance in NY supports the idea that the Celtics DID develop him as a player. He would not have his improved scoring ability otherwise, since D'Antoni has had no time to develop him.
Yet, the OP seems to have a different point in this thread - the Celtics did not develop value. This means that since his development was not seen during games, his trade value was below equal to his actual value.
This is all questionable, as Gerald had spurts as an effective scorer, as did Allan Ray. While it is not clear that playing Walker over TA or Quis makes any sense, there were many games when we were shorthanded at the wing where it seemed like a good opportunity for Walker. It is also important to remember that developing TA and Quis as cogs in this team was more important than developing Walker. Again, since we were shorthanded at times, it still seems like there might have been a few minutes available for Walker.
One problem is that playing a young player like Walker for 2 or 3 minutes at a time might not actually help him. In NY, he can play 30+ minutes with no pressure about defensive rotations. This helps him put up gaudy stats, making it tempting to overvalue him as a player.