Your eye test is not backed up by stats.
Baynes is top 30 out of 100 players in FG% for players with at least 1 postup a game.
His main problem is that he doesn't draw fouls, but there are plenty of players who have worse % in the post and do not draw fouls, including Deandre Jordan, Zaza, Favors, Kaminsky, O'Quinn, Gortat, Koufus, R. Lopez, and Plumlee.
I'm definitely not saying I would throw the ball into him more! I'm just saying the way he is currently used in the Celtic offense is reasonably efficient.
When you add in the fact that getting a player touches on offense helps to keep them engaged on defense, those touches are completely worth it, because Baynes is *cue Brad Stevens voice* really good on defense.