Remember, David beat Goliath, not the other way around
Nice posts by Tom Kane and Dan Hull about a topic close to my heart. Both talk about the fact that GCs do, in fact, hire smaller law firms.
I made the move from very large to boutique firm, so I have seen this from both sides. The best way for me to discuss this issue is to use a military analogy. Sometimes, you do need the Army and Marines. But sometimes, its better to use Navy Seals or Delta Force. There are strategic reasons to pick the small, elite force rather than the large force. Most cases don't require the large force, and hiring the large firm for the routine case is an invitation to overstaffing, overbilling, overlitigating.
I agree with Dan's comment that a client is far more likely to get high quality client service from a boutique than from a large firm. Small firms can much more easily create the institutional focus on service necessary to provide quality service.