Mark Hulbert, in the NY Times (April 27, 2005 at BU8), cited research that compared very successful to less successful companies in similar industries and circumstances. The “[less successful] companies … were far more likely to appoint white knights as their chief executives.” By contrast, the CEOs of more than 90% of the successful companies, those that transformed themselves from good to great, were promoted from within.
The same results likely play out in law departments: departments fare better over time, in terms of benchmarks like total legal spending as a percentage of revenue as well as qualitative assessments like collegiality and teamwork, when they promote someone from within. That person will know the company, know the qualities of former peers, and know what ought to be done to improve the department. A white knight starts in complete ignorance and quite likely faces resentment by those who were passed over.