Practically, I think these systems suffer terribly from inertia effects. The top stays the top, the middle stays the middle, because it causes so much angst to move someone from the top to the middle. So the middle just gets a job somewhere else.
GE's executive team claims otherwise[1]. "The top 20% and middle 70% are not permanent labels. People move between them all the time. However, the bottom 10%, in our experience, tend to remain there."
