You mean like a messenger bag? Personally I dislike carrying a bag or backpack for the usual reasons, plus the aerodynamic penalty. Any kind of box is much more convenient and useful in my opinion. Just tying a bag on the rack on the other hand, you don't have a rigid shape which makes a fairing problematic, and you have to fuss with securing it, ensuring it's waterproofed and keeping up with it, instead of just throwing stuff into the box.