The desktop experience team decided to try the bubbles in the middle right
of the screen, to avoid the problems with some apps' toolbars being
overridden. So, at the same time they reviewed the bubble placement to get
rid of the notifications stacking problems. When it was decided to put the
top right corner by default, the new placement stayed... and i'm personally
for removing it too. :)
As far as I know, the complete story is:
The desktop experience team decided to try the bubbles in the middle right
of the screen, to avoid the problems with some apps' toolbars being
overridden. So, at the same time they reviewed the bubble placement to get
rid of the notifications stacking problems. When it was decided to put the
top right corner by default, the new placement stayed... and i'm personally
for removing it too. :)