I'm pretty sure unleashing the WEAPONS was unintentional on Sephiroths part, there is no mention anywhere that Sephiroth even acknowledges their existence.
Besides, I think having Diamond in that position was just to keep the story moving and get the barrier down while making the part more interesting (Midgar and Rufus getting shot up, and the downfall of Shinra etc). Again, there is no mention that it is done to protect Sephiroth.