Companies are made successful by great people but the dirty little secret of successful companies is that they do not always hire great people, and firing employees can be tough for many people. It may be easy to fire employee whose performance ratings hover below six, the lower the easier.  But firing employees rated at six can be very difficult, bosses fear that they would be perceived as heartless, they may even personally feel guilty about doing something like that. But the fact is keeping such employees neither does justice to the organization, nor other employees. Most people would be appalled at the prospect of traveling in a plane that is piloted by a mediocre pilot or of their loved ones being cared for by a doctor/nurse whose performance rating is six. In order to maintain success-oriented workforce you need to do the weeding.
Source: NY Times