For some reason, I can't get 5 and 9 to make 16, but then again, maybe I should stop doing math on my fingers.
But yes, the flashbacks do add a lot to the movies. One of my personal favorites is Gandalf remembering his escape from Isengard. It was excellently done to let the watcher know what happened, but without taking up a lot of time.
Maybe it's just me, but the Smeagol flashback really creeped me out. I think it was the slow heartbeat in the background as Smeagol was murdering Deagol. But it did give more background on Gollum, and let everyone make a clearer decision on if they thought he was truly evil.
But my overall favorite is Gandalf vs. Balrog. That was just cool!