My main complaint about the weighing scale in the bagging area is the poor implementation for when you are using your own bag (which for me, is almost always).
Prompt: Are you using your own bag?
Prompt: Please place your bag in the bagging area and press OK to continue.
Me: Places bag, clicks ok.
Prompt: Unexpected item in bagging area, please remove item and try again.
Me: Gives up, drops bag on floor, then loads the bag once I've finished scanning and paying for everything! This of course takes longer, and so ties the till up for more time!
I suspect the issue is around weight, they expect you to put a carrier bag, like one of the bag-for-life ones, onto the till, when I always use a backpack if I'm on foot, which of course weights more.