The Wall Street Journal says that perjury that successfully covers up felonies should not be prosecuted:

WSJ.com - Obstruction for What?: Patrick Fitzgerald's investigation took nearly two years, sent a reporter to jail, cost millions of dollars and preoccupied some of the White House's senior officials. The fruit it has now borne is the five-count indictment of I. Lewis "Scooter" Libby, the vice president's Chief of Staff -- not for leaking the name of Valerie Plame to Robert Novak, which started this entire "scandal," but for contradictions between his testimony and the testimony of two or three reporters about what he told them, when he told them, and what words he used....

Mr. Fitzgerald has been dogged in pursuing his investigation, and he gave every appearance of being a reasonable and tough prosecutor in laying out the charges yesterday. But he has thrust himself into what was, at bottom, a policy dispute between an elected administration and critics of the president's approach to the war on terror, who included parts of the permanent bureaucracy of the State Department and CIA. Unless Mr. Fitzgerald can prove beyond a reasonable doubt that Mr. Libby was lying, and doing so for some nefarious purpose, this indictment looks like a case of criminalizing politics.

The "and doing so for some nefarious purpose" is in there because the Wall Street Journal knows full well that the evidence that Libby perjured and obstructed justice is very strong. Either Libby is guilty or:

1. An Under Secretary of State
2. A senior officer of the Central Intelligence Agency
3. The Vice President of the United States
4. Libby's own notes of his meeting with the Vice President.
5. A briefer from the Central Intelligence Agency.
6. Libby's then-principal deputy.
7. Judith Miller.
8. Tim Russert.
9. The White House Press Secretary.
10. The Counsel to the Vice President.
11. The Assistant to the Vice President for Public Affairs.
12. "White House Officlal A".
13. Matthew Cooper.

are lying. Libby's story is contradicted not just by a few journalists, but by his own notes and more than a half-dozen senior administration officials as well.

Hence the Wall Street Journal's declaration that if the obstruction of justice is successful--if it keeps the prosecutor from being able to prove the underlying offense beyond a reasonable doubt--it should not be prosecuted.