Reality:
Duncan does actually produce on about 50% of his touches. He either gets a score or draws a foul, or passes back out of the post- with no loss of possession, about half the time he the posts up. And many times when he passes out of the post it results in a score or a shot and a recovery by the Spurs!
This is what makes the Spurs so difficult to stop, even when TD is slightly below average in the playoffs. He is so automatic, and so difficult to stop that the Spurs can continue to post him up and run plays off his posting up, and have a better than average chance of success.
Futher, the mere threat of TD in the post is what opens up all those outside shots for Finley, Horry, etc. And, to a certain extent is a limiting factor in Parker's assist totals.