The point is that either the watermark makes the image unusable or it is easy to cut or clone out. In two of the recent cases of online image theft the watermark was in fact cloned out.
http://digg.com/tech_news/Stealing_IS_a_crime_right
http://www.flickr.com/photos/larajade/513641346/
Of course everyone is entitled to their own opinion on watermarks but unless they are done in such a way as to make removal difficult they will not stop someone from taking the image for their own purposes.