Last year we caught Dave Letterman reusing the exact same jokes (gestures included) just days apart. Last week he was at it again, repeating the same tired punchline Friday as the one he delivered Tuesday.http://www.huffingtonpost.com/2009/02/16/letterman-caught-recyclin_n_167312.html
Most people are in no way funny. Who are they to complain?
Jokes are one's property. You trot them back out now and again for various reasons.
Incidentally, even though I fall to my knees in awe of this Glorious New Golden Age of Comedy which promises to terraform the earth and bring rainbows and woodland elves and gnomes to the world, I never meant any slight to David Letterman or Tina Fey or Chris Rock, all of whom I highly regard.
I've been watching David Letterman since he had his morning show back on around 1980. The man is infinitely funnier than your average person. If he wants to recycle some old crap for an audience that he probably despises anyway, that's his prerogative.