I am not worried too about someone else messing with my stuff going in but more of knowing where it is at. For instance, I travel with a pack so part of my SOP is making sure straps will not get hung up on anything. Often TSA will hold up items so to rescan something. If that is my pack I can make sure the straps are okay. At the same time, scanners get backed up. I would rather wait and watch my items go in so that once on the secure side I only need to wonder where there are in the scanner. And given I am usually through the WTMD I can wait to be assured no on the secure side messes with my bags. At the end of the day it is all about control.