The reason Libby is arguing he was put forth as a sacrificial pawn to protect Rove is so that the defense can impeach the prosecution's witnesses on their alleged motive to lie to protect Rove.  The also need an alternate narrative.  You can't win by just poking holes in the prosecution's case, you have to present a believable theory as to why the facts are as they are.  Unless you are O.J., of course.