I do feel that coloring books limit creative expression. I think children should have the opportunity to create independantly.
I noticed, when teaching 4 and 5 year olds, that students who had been consistently exposed to coloring books felt there was a "right" way to draw. Many of these little ones would go as far as to refuse to draw, bursting in tears that they "can't."
Students that have been given the freedom to draw, color, and paint without any direction are much more autonomous! They general feel they can do just about anything including draw. They jump in fearlessly.