I do not know if the problem I describe hereafter exists, but the risk of a perception of impropriety exists.
A senior lawyer in a department directs substantial sums to a law firm in the year before the lawyer retires or leaves the department and joins that firm. That coincidence may be benign, or it may be suggest improper decisions. Whichever, might a cautious company add a prohibition in the terms of employment of any lawyers who selects or approves bills of a firm? Any lawyer who did so may not join that firm within one year of leaving the department (See my post of March 8, 2006: do soon-to-leave general counsel favor certain firms.).