Thinking though how I use UIs, I like having one sidebar per screen edge because:

- I can spatially separate OS-level and program-level controls instead of visually/mentally hunting around in the same general area of the screen

- The effective clickable surface area of a button next to a screen edge is effectively infinite. For example, you can "crash" the mouse pointer downward onto a minimized window such that the bottom edge stops your mouse pointer from overshooting, and clicking will maximize that window. If you stacked OS-level and program-level control bars onto the same edge of a screen, one of them loses this UI perk