Among the advantages of George Bush's decision to commute Scooter Libby's sentence -- and not pardon him -- is that it keeps Scooter's appeal of his conviction alive, and therefore maintains the 3 year silence from the White House concerning the leaks that originally prompted the investigation. Because any questions can still be dismissed on the grounds of refusal to comment on "an ongoing legal process."
Note also the role of this Washington Post op-ed from nearly a month ago in the kite-flying for the commute option. Note the provenance of the author, William Otis, classic "Wise Man of Washington" type (and, apparently, with a Powerline connection).
UPDATE: The statement of justification from the White House for the commutation.
FINAL UPDATE: Our initial line of argument above has been around for a while.