One gets a good rating for fighting a fire. The result is visible; can be quantified. If you do it right the first time, you are invisible. You satisfied the requirements. That is your job. Mess it up, and correct it later, you become a hero.
The idea of a merit rating is alluring. The sound of the words captivates the imagination: pay for what you get; get what you pay for; motivate people to do their best, for their own good.
The effect is exactly the opposite of what the words promise.
Motivation - nonsense. All that people need to know is why their work is important.
Top management should publish a resolution that no one will lose his job for contribution to quality and productivity.