The knock on general counsel who reflexively call Prestige, Admired & Realcostly to gain cover if things go wrong goes might be called CYA Theory (See my post of Aug. 15, 2008: CYA and 6 references.).
Some truth there probably is to that theory, but another reason to pick up the phone to BigFirm is it can probably find someone faster than a smaller firm who knows more about the problem. Speed of response counts too, not just the billboard repute of the firm. Specialization and reputation go hand in hand, of course, so it makes sense that a general counsel under pressure, someone who needs guidance or a check for a tough call and needs it quickly, will favor the larger firm with its deeper, and more quickly available, knowledge base