Yup - their team defense is tremendous. Duncan is a great individual defender, but it takes a heck of a lot more than great individual defenders to be a great defensive team. It's coaching, and the players buying into the system. I mean, outside of Duncan/Parker/Ginobli the Spurs players range from a little above average to below average. And even Ginobli these days isn't great like he was several years ago. First and foremost their success comes from Pops, then a combination of the Big Fundamental and the constantly attacking Parker. Remove Duncan or Parker and they're still a good team, but not a contender. Remove Pops and that team would be lucky to make the Playoffs.
