I can say that I have known some people to be real pains in the rear, but they were some of the most ethical traders I've come across. On the same note, some were as bad a traders as they were individuals.
Being able to change a rating would allow you give some one a second chance. If a person were to change their ways (for better or worse), it would be handy for the reevaluation of the member. It could serve as a praise to those who improve, or warning to some one if they were to start slipping in their social skills and or scruples.
Added note- Paulson, I hope my rating of you improves your ranking. I have found an occasional post of yours to be helpful. I just can't figure out how you find time to post so many.