Terry Michael thinks that Chuck Hagel should prefer a pose of ideological purity to becoming secretary of defense:
Assuming his nomination isn't proactively yanked by the president, here's the question that Hagel first needs to answer: Should he allow himself to be used as a pawn in Barack Obama’s continuing deflection of presidential responsibility?
Tempting as it may be to get inside the tent, Hagel should decline. Given Obama's uninspiring track record, he won't have a major impact on policy. Far more likely, he'll serve as a prop for a president who asserts the right to kill even American citizens without judicial oversight and to send manned and unmanned planes anywhere he chooses.
This doesn't sound very persuasive. Ultimately, if people sympathetic to Hagel's views on various matters want their arguments to prevail, they're going to need to wield positions of authority. That will naturally entail some compromises and deviations from time to time, but that's inevitable. An ideological purity that never translates into policy outcomes isn't worth much.
Update: Larison adds more:
What makes Terry Michael’s argument even less persuasive than Scoblete allows is that Hagel would have virtually no influence as an outside critic demanding that Obama “bring the troops home now.” There is no guarantee that a Secretary Hagel would move the administration in the right direction on many things, but he will likely be a brake on military spending and future military action instead of a goad to both. The chances are good that he would be an advocate for withdrawing U.S. forces from Afghanistan as quickly and completely as possible, and he stands a much better chance of making that happen if he is inside the administration than if he remains outside it.