Comping skills are still in high demand for story boarding commercials and scenes in motion pictures or to conceptualize something that doesn't yet exist. Comping has become a specialized skill set. Not many designers can do it well any more.
To do thumbnails and comps in a design process is a way for me to reinforce in your mind that YOU are the designer, not the computer or any of the technology currently utilized today. Computers will not make a better design just by using them. There is no "Make good design" button on the keyboard or in the drop down menus. You are the creative factor and you can be creative and design with or without computers. A good design hinges upon creative application of the basic design principles and elements. Those principles and elements should also be included on the tatoo list.