You don not buy heroism, just as you can not buy respect. Human capitol will be much easier to recruit if the recruiter represents an entity respected by all. That image is a direct result of Congressional Oversight. While the congress can delegate its authority, it can not delegate its responsibility. Everyone wants, in their heart, to work for men and women that are universally recognized as leaders.
Reputation is everything.
Secretary Powell does not carry with him the reputation of being just another bureaucrat. He does not carry with him the stigma of just another politician working toward re-election. He is a leader that comes with the reputation of proven sacrifice for the nation, demonstrated skills, and projects the image that he will work honestly to improve whatever project he is given. The typical benefits and general HR considerations are important. But they do not bring the best our nation has to offer.