All companies love boxes for 'security' but not having one is best for awkward/variable sized/overly long or wide deliveries. plus it nice not having the aerodynamics of a brick.
That makes sense. I quite often end up riding with bigger bits strapped to the top of the box. Not ideal when you have to peer round the side because you can't see over the top.
The Bullitts I use for work all have metal boxes on the front rather than bags. What's the advantage of the bag? Mostly weight?