This had been added in 1.8b3. Was not present in 1.7.3, 1.8b1 or 1.8b2.
It is kind of linked to Nav Items Div is not <UL> as standard (sort of). If this was made a hard coded <ul> then display: list-item; is redundant as we would be using proper semantic HTML structure with <ul>'s & <li>'s.
Is this really required and if not can it be removed as it causes some display issues such as not being able to set an order for menu items. Useful when wanting to change the order when in mobile menu etc
EDIT - missed a screenshot out
