It is never good to measure the "one" metric and manage to that, it results in people who game the metric "winning" and fosters the promotion of such behavior. I pointed this out to Google leadership and to quote Lazlo, "This is the system we have, it isn't perfect but we're going with it, either work within it or don't, it is up to you." :-). That meeting told me everything I needed to know about whether senior leadership was trying to have a better engineering environment or not.
To many new managers come into the job (especially if they were engineers individual contributors before) with the idea that if they keep the "best" members, and move out the "bad" members both team morale will be great and so will the teams output. But they miss that their understanding of "best" is not based on managing people, its based on getting their original job done. As a result they favor people who mirror their skills and habits and disfavor those who have different skills and habits. Getting them to see this and having their eyes go wide with realization is always interesting.