Duncan wasn't a good closer? I'd say he wasn't great, but he was most definitely good.
http://www.82games.com/gamewinningshots.htm
Of the data between 03 and 08, Duncan was 0.391 (9 for 22) on game closing shots, with a FG% rank of #6 out of the players with the top 22 (8 players were tied for the top 15) made clutch FGs.
During the same stretch, Manu made 6 of 21 shots for 28.6% (not in top 22), Nowitzki made 12 of 37 for 32.4% (13 of the top 22), and Garnett was 9 of 33 for 27.3% (19 of the top 22. Only Joe Johnson, Jamal Crawford, and Kobe Bryant were worse).
In the playoffs, Duncan and Ginobili were both 2 for 6 in the clutch stats.