A piece in Managing Ptr., July-Aug. 2010 at 49, explains how a major British law firm places its clients into a 2 x 2 portfolio matrix. The matrix sorts clients by attractiveness, which means either to strategically invest in them or simply maintain them, and by whether the firm has a competitive advantage relative to other law firms in serving them. The implication is that the firm will drop or stop investing in the less attractive clients or those that seek non-strategic services of the firm.
If law firms actually prune such clients, we should stop thinking of convergence as being driven solely by aggressive law departments. If law firms deliberately end some representations or milk them as cash cows, then the much touted reduction in the number of law firms retained by law departments represents a winnowing process at both ends of buyer and seller.
I doubt that individual partners in law firms willingly surrender any amount of work for any client so that this is more of a theoretical possibility than an actual cause of reductions in the rolls of clients.