The LA Times says it well
… people in the Bush administration misused an intelligence secret to discredit a critic of its Iraq policy. And outing Plame, whether illegal or not, did harm to our national security. Plame may work in Langley, Va., but she worked with others who work in more dangerous locales. You only need to imagine how Republicans would have treated such a leak in the Clinton administration to dismiss their protestations that it's all no big deal.
It's a good bet that there has already been some lying under oath.
Perjury is your classic coverup method, and still is used when other methods have failed. Advances in the science of spin since Watergate, however, have made a high-risk, Nixon-style coverup unnecessary in many situations.