Harvard Business School's "Working Knowledge" has a post, Why Isn’t ‘Servant Leadership’ More Prevalent?, which is a bit strange because the article never addresses the question. However, I did think this was interesting:
Now it appears that a group of organizational psychologists, led by Adam Grant, are attempting to measure the impact of servant leadership on leaders, not just those being led. Grant describes research in his recent book, Give and Take, that suggests that servant leaders are not only more highly regarded than others by their employees and not only feel better about themselves at the end of the day but are more productive as well. His thesis is that servant leaders are the beneficiaries of important contacts, information, and insights that make them more effective and productive in what they do even though they spend a great deal of their time sharing what they learn and helping others through such things as career counseling, suggesting contacts, and recommending new ways of doing things.
Further, servant leaders don't waste much time deciding to whom to give and in what order. They give to everyone in their organizations. Grant concludes that giving can be exhausting but also self-replenishing. So in his seemingly tireless efforts to give, described in the book, Grant makes it a practice to give to everyone until he detects a habitual "taker" that can be eliminated from his "gift list."