Shadows in pictures can be very interesting and work in different ways to enhance your pictures, but they can also ruin pictures. The third picture down shows both, to me it ruins the picture but to the photographer it enhanced the photo. I think there must be a story or a purpose behind the photo.
In photography class offered here at the high school we battled shadows. Many photos were usless because of diferent shadows blocking different things. The biggest problem with shadows I found was when I was trying capture an optical illusion. I also had problems with capturing portraits outside without shadows across the face or body. Shadows are difficult to work with.