Powell provided a voice that Rice may not have

There is a certain irony that Colin Powell, almost alone among the president's top advisers in correctly counseling against the war in Iraq, is now being replaced by Condoleezza Rice, who was wrong before the war, during the war and after the war.
It was Powell who invoked what he called the Pottery Barn rule in warning the president about Iraq: if you break it, you own it. Now, with more than 1,200 U.S. military personnel dead and thousands more wounded, the United States is still paying the price for invading Iraq, but it hardly owns it.
Powell remained the good soldier up to the end, not publicly criticizing the president, staying on through the election and doing enough busy work that his absence from the campaign trail on the president's behalf could be plausibly excused.
Powell even made an impassioned defense before the United Nations of the need for the United States to invade Iraq. He didn't even make a scene when it subsequently became known that much of the most damaging information Powell presented had been cooked by Vice President Dick Cheney's office.
He dared to disagree
Still, it is clear that Powell dared to disagree with the president at least privately and tried mightily if unsuccessfully to steer the administration on a different course, especially regarding Iraq and North Korea.
The American public cannot be as confident that Rice will exhibit that kind of independence.
As national security adviser, Rice ignored repeated warnings that Al-Qaida represented a real threat to the United States. After 9/11 she excused that shortcoming by saying that no one could have imagined terrorists using an airplane as a weapon. She misrepresented the purpose of aluminum rods imported by Saddam Hussein, stating emphatically that they were evidence of Saddam's nuclear weapons program. And she joined almost everyone in the Bush administration in underestimating the troops that would be needed in post-war Iraq to establish order.
Rice is unquestionably a brilliant woman, but we do not see how her record as national security advisor qualifies her for a promotion to secretary of state, where her voice on all foreign policy issues will be second only to the president himself.
Unless the president values her ability to make him feel comfortable about his choices more than he would value an adviser who challenge him.
Arguably Powell was not a good choice for secretary of state. Not because he was incapable of doing the job, but because he and the president were too often reading out of different hymnals.
The choice of Rice could be an even bigger error, though. She and Bush are not only reading out of the same hymnal, they're on the same hymn, same word, same note.