I have a simple theory on that. Art is like a language and one should use what serves him best to express and forge what he feels what one wants to create with. Same goes for the tools you use. In my oppinion, illustration and todays cg art, craft and knowledgewise still needs all the same the old masters were working with (composition, perspective, color, anatomy, visual storytelling...). Painting digitally is just the use of a tool, the craft and skill behind it needed to provide a solid work still is the same like when you paint with oils or acrylics or even draw with a stick in sand. What if - Leonardo, Rembrandt or whoever - would have gotten a Cintiq today - the chance to paint with pure light - don't you not think they would have enjoyed that tool and painted on it with lots of fun?