Agree. You cannot mistake activity for achievement. But hustle is a skill, and when combined with above average rim protecting numbers, Zeller ranks in the top 15 in the NBA in points saved per 48 minutes.
I think his rim protecting number is pretty average (currently at 50% and sitting pretty much in the middle of the pack). So he's average at challenging shots, but hustles a lot (55% is a top ten rate). I guess that's better than an average "shot challenger" that doesn't hustle a lot, but still not much to write home about.
I like Zeller. He just won't be confused for any sort of defensive force in the paint right now.
Think about it like this: Player A contests 10 shots at the rim a game, holding drivers to only 4 makes with 2 BPG. However, Player A is sometimes nearby but fails to contest 5 more shots a game-all makes. His OFG% at the rim would be 40%, but the scheme dictates that he should have protected the rim five more times and failed to do so. In other words, in reality, he is allowing 9/15 drives to score, or about 60%.
Player B contests 13 shots at the rim a game, holding drivers to 6 makes with 1 BPG. Player B is sometimes nearby but fails to contest 2 more shots a game-both makes. His OFG% at the rim would be 46%, but the scheme dictates that he should have protected the rim two more times and failed to do so. In other words, in reality, he is allowing 8/15 drives to score, or about 53%
Activity does not mean everything if Player B fails to prevent the shot from going in, but Player A's BPG and OFG% can look better than Player B's simply because it is difficult to quantify a lack of effort.
I would always choose an average player with good hustle over a good player with average hustle. You can coach skills. You can't coach heart.