Glued to the bench? Well, the first year he was injured for critical stretches and couldn't really get into the row Tatian because he didn't know the system. Then pop went to him when we had no other option and although he was raw, he performed well. I mean in the Memphis series. Then last year, again with very little opportunity for off-season development due to the lockout, he performed at an extremely high level at 19 minutes a game as you said. His efficiency rating is at about 21 point something per game, which puts him right about the top 10 in his position across the league. That is not a guy who is glued to the bench or a guy who is a bust. Now he has played very well off the bench against opposing bench players and contributed greatly to our offense of efficiency in the second unit.
That in mind, I don't want to say he's the best thing to come along since perforated toilet paper, but he is playing very well and has room for improvement. In my opinion, he is extremely skilled and intelligent, but not physically tough or imposing. He takes charges and rotates pretty well, but at times in the playoffs he showed a level of confusion and lack of hustle that was disappointing. He definitely needs to improve, but even without improvement, he is not a blast and he is a very highly skilled big man.
Short version: he is an extremely smart and skilled big man who helps out our offenses efficiency in the second unit like probably no other big man in the league does for his team, but he is not a shot blocker or grade rebounder or intimidating physical presence, and has a lot of work to do with mental and physical toughness.