I think the point you bring up misses the whole strength of his game: he is a center. Especially this year he is playing defense and not giving up anything there, then having his skill against the opposing centers. Even without a scorers mentality he makes an impact. Having that space and passing on the floor makes a difference (numbers bear this out for him).
I believe that
was my point.
He gives you small forward production at the center spot without killing your defense. That's got value in an of itself because of how it adds skill and shooting at a position that normally doesn't feature those things.
If people learn to accept that
that is what Kelly brings to this team, not the promise of a seven foot top scoring option, or anything resembling a traditional big man, they might feel less frustrated.
Then again, if like Koz suggests, you need your big men to rebound the ball and mix it up inside, you're gonna feel frustrated.
And by the way, yes, it's true that Kelly plays fewer minutes than some of the other guys listed. Regardless, his production and role in the Celts' offense is similar to how those guys play. If you look at the game logs for those players, I think you'll find that they go through similar ups and downs in production. That's what happens when you're a role player on the wing.
All of this makes sense, too. Kelly was a smaller player through most of his time learning the game. He didn't shoot up in height and start playing in the frontcourt until later on. His mindset, skillset, and overall play style still mark him as a big guard or combo forward. He just happens to play center.