I agree - a slip wouldn't make any sense with the dress. All it would do is mean that, instead of seeing her legs (gasp!), you'd be seeing the slip, which certainly wouldn't be a good look.
In general, it seems to me that Kate's long hair and long legs are traits of hers that she seems to like. I think many (if not most) people have certain aspects of their appearance that they favor, and I think sometimes people stick to those highlighting those things because they might feel they detract form other areas that they find less flattering. Much as it drives some people crazy, Kate seems comfortable with her hair and her legs, and I don't really anticipate her to make any changes in those areas any time soon.