Actually, NOT so rare at all since 2002 and 2007 yielded no decent QBs at all, and depending upon how you rank Carson Palmer and Jay Cutler, 2003 and 2006 weren't very good either since those were the only QBs who amounted to anything from those drafts.
What actually IS rare is getting a true "franchise QB" who performs at a high level most of his career. What's a once in a generation occurrence is getting more than one from the same draft. Consequently, I'm NOT optimistic that the 2011 and 2012 drafts will actually yield multiple "franchise QBs". If there's one in the lot, that's a good draft.
First & Second Round and other "successful" QBs from other rounds since 2000:
2000 - Chad Pennington (#18), Marc Bulger (6th),
Tom Brady (6th)
2001 - Michael Vick (#1),
Drew Brees (2nd),
Quincy Carter (2nd), Marques Tiasosopo (2nd)
2002 -
David Carr (#1), Joey Harrington (#3), Patrick Ramsey(#32), David Garrard (4th)
2003 - Carson Palmer (#1),
Byron Leftwich (#7), Kyle Boller (#19), Rex Grossman (#22)
2004 -
Eli Manning (#1), Phillip Rivers (#3), Ben Roethlisberger (#11), JP Losman (#22), Matt Schaub (3rd)
2005 - Alex Smith (#1),
Aaron Rodgers (#24), Jason Campbell (#25), Kyle Orton (4th), Matt Cassell (7th), Ryan Fitzpatrick (7th)
2006 -
Vince Young (#3), Matt Leinart (#10), Jay Cutler (#11),
Kellen Clemens, Tarvaris Jackson (2nd)
2007 -
JaMarcus Russell (#1), Brady Quinn (#22), Kevin Kolb (2nd), John Beck (2nd), Drew Stanton (2nd)
2008 -
Matt Ryan (#3), Joe Flacco (#18),
Brian Brohm (2nd), Chad Henne (2nd)
2009 - Matthew Stafford (#1),
Mark Sanchez (#5), Josh Freeman (#17)*,
Pat White (2nd)
2010 - Sam Bradford (#1)*,
Tim Tebow (#25)**, Jimmy Claussen (2nd)
I bolded the guys I think are "franchise QBs", the others are "useful" QBs, from decent starters to very good starting QBs. I put the outright busts from the first and second rounds in red (IMO, a QB drafted after the 2nd really can't bust). The guys that are marked as * are questionable in that they are young and haven't seemed to progress as much as hoped. I think if they don't step up this season, though, they're probably busts. ** for Tim Tebow is that he shouldn't have been picked in the first round. What was Josh McDaniels thinking???
These kids are still works in progress ...
2011 - Cam Newton (#1), Jake Locker (#8), Blaine Gabbert (#10), Christian Ponder (#12), Andy Dalton (2nd)
2012 - Andrew Luck (#1), Robert Griffin III (#2), Ryan Tannehill (#8), Brandon Weeden (#22), Russell Wilson (3rd)
... Some are shining early on, but the key to being a "franchise QB" is to play well over most of your career, NOT for a season or two. The book is still out on these young QBs.