When stressing an aircraft we stress to a lot of loadcases not just too much weight but manoeuvres and various other stages of flight. So with regards to the amount of weight in the luggage bins, the danger of the whole bin coming off only come into play in extreme circumstances and not during reasonably normal flight operations (a firm landing in the aircraft during normal ops is very much reduced from what is actually accounted for or tested for during trials, I believe there is a video clip on the internet of a DC something during such trials that snaps the fuselage).
A lot of the discussion here it seems is coming from whether people are as special as they think they are. In general, no, each person has paid for a specified level of service and allowances, to exceed these knowingly is being pigheaded if you don't expect to pay for it. Otherwise pay up and get on with your life.
Back to the original question about scales, I checked two bags in one being 52 lbs and the other being 47 lbs, I was asked to move 2 lbs from one to the other to make them within limits. Fine, I pulled them off the scales and shifted 2 lbs from the heavier to the lighter. I put them back on the scales the first one weighed 51lbs and the second weighed 52 lbs! Somewhere in the minute it took to transfer the weight I had found an additional 4 lbs.