At a SuperConference session on litigation management, a lawyer from CapitalOne said that his department allocates a portion of its spend on outside counsel to firms that are not on the preferred partner manifest. He said “It’s partly training them and partly assessing them to see if they might become a preferred firm.” Those are good reasons. Reasons with more bite could be added.
As I see it more competitively, no law firms deserve to feel entitled to a flow of work, locked in, guaranteed a place at the trough. Legitimate competition waiting in the wings keeps everyone honest. For that reason, give some work to contender firms.