Centering the subject has often been seen as a beginner technique, because the resulting composition is not viewed as being as dynamic compared to other compositions (for example, last weeks' Rule of Thirds). However, if you are deliberate with your composition, having a balanced frame with the subject at the center can be a very effective compositional technique.
A center composition can emphasize symmetry or create a sense of space. If there are distracting elements around the subject, centering may be the best way to avoid including these extraneous elements in the frame. When there are very few other items in the frame, centering can draw attention to the subject, especially when leading lines are used. Centering is particularly effective in a square crop. When used purposefully, centering can produce a very visually striking image. Here are all of this week's center composed images: http://whowebecome.com/?p=2118