Bass is a very good defender against sf's and undersized pf's that play on the perimeter.
How do you figure? Despite what all of the people wearing Celtics glasses will tell you, Bass' defense against Lebron in the playoffs was horrendous. Lebron went past him like he was standing still and pretty much scored at will. Pietrus, Pavlovic and Marquis all played better defense agasinst opposing SF's than Bass did and they are all veteran minimum players.
Bass got his contract for these reasons:
1. His midrange jumper
2. His work ethic
3. His youth
His defensive rating has been consistently poor ever since he's been a Celtic, completely regardless of whether he started or came off the bench, and which players he played alongside.
He's capable of playing good individual defense but only against undersized PF's or slow SF's.
His team defense is terrible.
He has zero post up game
He has no range outside 15 feet
He's an average (at best) rebounder
With his jumpshot falling like it did last season he's barely worth his contract. With his jumpshot not falling he is worth the BAE at best.
The truth is somewhere in between.
Bass has never been an elite defender against SFs. He tried his best against Lebron, but he just didn't have the skill set to defend the best player in the world.
However, last year his defense was really quite good. Last year, Bass ranked 16th in the NBA in terms of points allowed per possession. He ranked 14th in isolation situations, 7th on post-ups, and 2nd covering the pick-and-roll man. That's really excellent defense. A lot of it was the KG factor, but overall Bass did a really good job.
This year, he's been much more mediocre, and his decline has been a big part of our early season struggles.
Also, in terms of the statement "He has no range outside 15 feet", that's simply not true. Last season, Bass was most prolific shot was from 16 feet out to the 3PT line.
Link.