In this example, I came across two articles where I felt was grossly inaccurate. While the originating site allowed feedback to be submitted, I wasn't sure if they would publish my comments so my blog allowed me to post my views and share with others. You can also see from the post how others post their reactions in my blog, thus giving a more balanced picture to the original post.
I also deliberately chose to use the same article title for my blog post, so that when someone googles it, my blog post might be picked up with the originating articles:
Brave New Singapore?
Learnt from Loy's blog (From a Singapore Angle, 7 Jan 06) about a two-part series commenting on "the lack of liberty in Singapore" ...
Personally, I think expressing one's opinions & reactions in one's own blog is a more mature position to take, as opposed to flaming someone in their articles or posts. :)