Adding too much detergent to the washer for the size of the load can be harmful to the washing machine and leaves soap remaining on the clothes. Using more soap than you need costs you extra money by shortening the life of your machine and uses up your detergent faster. Another factor to consider is the resources used by municipal water treatment facilities to remove soap that is added to the water. Any detergent that leaches into the environment is toxic to aquatic organisms and can lead to algal blooms.
For more details, see this article at earth911.com.
Photo by Susan O'Donnell