The problem with this is that it enables weak-willed managers to claim that everyone on their team is great, serving as a kind of grade inflation that makes true high-performers seem less impressive. As I understand it, the curve is only enforced at something like the VP level, so it's not like a team of 10 great engineers is going to be forced to have 2 people scored as underperforming. Are there 200-person groups at Microsoft where everyone's better than average?