Count me as skeptical. Ratings of past jobs only work for people who do one project per month, otherwise a single dispute distorts the rating. It's prone to extortion and manipulation. You do that feature for free or I'm going to ruin your score. People will spend lots of time massaging their online profile instead of getting work done. Lots of useless blogs, tactical contributions to open source projects, etc.

The only way to find out whether someone is good is to see first hand what he/she has done before or work with them on something small and then build on that.