DS's prep teacher last year told me that the scribbling thing is actually an important first step in learning to write real words - if a child scribbles something on a page and attaches a meaning to it, then they are starting to learn the concept of writing and also the fine motor skills attached to that, like pencil grip. She would actually encourage the kids to do it - after they had painted a picture, or drawn something, she would ask them to "write" down what it was, and she would write underneath what the child had told her it means. By the end of the year, the kids were writing real words along with their scribble - as their knowledge of letters and how to form them grew.