There were almost 100 guys picked from 21-30 the past 10 drafts. Only around 15 have become better than average rotation players. It's easier to find a solid rotation guy on the veteran scrap heap than through the draft.
Players drafted in 2002 or later in spots 21-30 who have had at least two seasons (counting a partial 2011-2012 as a full season) with significant minutes of .110 or more WS/48 (0.100 is supposed to be approximately average):
Tayshaun Prince, Kevin Martin, David Lee, Rajon Rondo, Josh Howard, Leandro Barbosa, Delonte West, Kyle Lowry, Kendrick Perkins, Beno Udrih, Nenad Krstic, Tony Allen, Jared Dudley, Ryan Anderson, Nate Robinson, Nicolas Batum, George Hill, Jason Maxiell, Sasha Vujacic, Luther Head, Serge Ibaka, Linas Kleiza, Taj Gibson, Jordan Farmar, Trevor Booker, Tiago Splitter, Rodrigue Beaubois
Notable players who did not make the cut: Boris Diaw, John Salmons, Jarrett Jack, Carlos Delfino, Aaron Affalo, Francisco Garcia, Travis Outlaw, Rudy Fernandez, Courtney Lee, Aaron Brooks, Wilson Chandler, Darren Collison, Shannon Brown
I've named 40 of the 100 players drafted 21-30 in the last ten drafts and I've left out some guys such as Brian Cook. Plus, more recently drafted players such as MarShon Brooks haven't had time to have two such seasons.