I don't have any particular experience with search firms, but there could be some advantages to going outside of your own networks in making those kinds of searches.
One reason we often see a lack of diversity in something such as a board—or any group—is that often these things are selected from within your social networks. You see a position open and ask, who do we know who would be good to fill that position?
When you go to a search firm, the search firm is going to look beyond your social networks. If they're given appropriate directives to look for a diversity of views, they may do better than your own networks in finding some very good, competent candidates for those positions.