If a project is launched from SI by selecting it in the main projects window, going to the "Open" menu in the top left of the task bar > Visio > Open In Visio, the Project Editor window loses the "Drop Zone", location pane, and system pane.
This is very cumbersome to deal with. Oftentimes, if there's a need to work in an old project and start cleaning it up (i.e. Location & System assignments, accessorizing items like power cords, etc), you won't know if you're grabbing the right items without having the drawings open to double check that Component ID XX is actually at the lectern as opposed to the identical device with Component ID YY at the rack, for example.
However, having the Visio drawings open removes those assignment panes, so now you have to select + right-click + Assign every single item (or select + use Assign tool on taskbar). On the flip side, if you only have Project Editor open with no Visio, these panes are there and it's very fast to clean up a project, but now you have the ability to work quickly without the reference material (Visio drawings) you need to accomplish this work.