Is it a good practice to combine small portions of content from other sites?

On this video, Matt Cutts try to answer one question of Webmaster Help Forum by Google related of some people parctice of include lots of excerpts in his/her posts.
Matt says that he would avoid this practice. He would recommend the way Wikipedia works, i.e. sinthesize information of several pages with your own words and refer to the links at the bottom of your article.