I know your post was a couple of days old but I feel like this is an important point to reiterate that I haven't seen others mention.
Those steps back to my vehicle were always done while breaking down the previous casepack. So it wasn't totally unproductive time like some people make it out to be. It's really a comparison between walking while breaking down box or breaking down box in a space restricted area and having to move for almost every guest.
Now, sure, some people might walk extremely slow. Others might be "more tired" at the end of a shift from the extra steps. But for those of us who are used to moving quickly, even having to move the uboat a couple of times makes the overall process slower than breaking down the box while walking to retrieve the next. Best case scenario, with no guests, it would be the same speed as compared to 1 vehicle at the end of the aisle, but still probably slightly slower than having a 2nd vehicle for trash and backstock like I used to. (Having to work around your trash and backstock on the same vehicle is a pain in some areas with a lot of trash, and in most areas with really full uboats)