In-house managers of outside counsel want as few lawyers as possible at a firm doing their work. The benefits of a core of dedicated lawyers are well known: familiarity, and competence, confidence. Work with a small group and you work more effectively (See my post of Aug. 8, 2006: core staff with 6 references; and July 17, 2008: core team with 11 references and citations to 7 earlier; and Dec. 27, 2008: timekeepers with 18 references.).
At the same time, with a pull from the opposition direction, legal departments face the need for continuity and new blood. Departments need bench strength at their primary firms so somehow the core team needs to admit newcomers. The tension boils down to the question of who pays for training backup lawyers so that they can fill the shoes of the core team members.
Ultimately, the law firm should bear most of that cost of training, not the client. Attrition is high at law firms and the client may never recover its investment.