The real advantage of the header dropdown mode (for both the basic user and admin) is that it has two columns, which significantly improves the menu experience, makes things clearer to see and reduces the vertical navigation required.
imho, the amount of options and scrolling involved in the single column sidebar makes for a less pleasurable and practical experience.
I really do feel that we shouldn’t get too fixated on “having a sidebar for the sake of having a sidebar”
Happy to raise a feature request to both retain the Header Dropdown mode permanently and to support the admin menu in that format.