Why is this being examined as though it were an important political face-off? It was a late night talk show appearance. And I don't think either man behaved out of the ordinary. There is no winner or loser.
Did Letterman say that he "has a feeling that 60% of what Bill says on the Factor is crap"? Or did he say that he "has a feeling that 60% of what Bill says" is crap?
Letterman admittedly does not watch Bill's show. Letterman is probably very well informed, especially on matters of importance, but he's never been remotely political in his public or professional life.
Bill, who fancies himself very political, before he even sits down, begins with the Christmas stuff, which Letterman rightfully sees as silly. Bill proceeds to spout a number of incidences that all sound a little fishy to Letterman, though Letterman can't refute them outright, because to him it's a non-issue and he has not looked into it in the least.
They then discuss Bush and Sheehan, and Bill uses the fact that Sheehan called insurgents freedom fighter (which strictly speaking they are, they just happen to be fighting for freedom from Americans) to get some cheap applause.
Letterman, preferring jokes and laughter for his show to half-baked rhetoric and warmed over spin, recognizes this further helping of BS and says (most likely based solely on what he's gathered from Bill's short visit) "I have a feeling that 60% of what you say is crap".
Both men were served for an entertaining talk show apearance, and that's it.