Originally Posted by N1Rotate

The safety issues of carry on luggage are not only to do with them falling on your head but also aircraft performance and mass and balance considerations. Airlines typically use assumed weights for hand luggage (an assumed weight for a passenger and their hand luggage) and when you have a bigger cabin baggage allowance you increase the variability of the actual weight of cabin baggage and lower the accuracy of the assumed weight. If 100 passengers take 10kg extra of hand luggage that is not accounted for it could make the difference between hitting or missing an obstacle in an engine out scenario after take off. If an airline decides to use a higher assumed standard weight it would end up having to unnecessarily leave behind passengers and cargo, QR's US-DOH flights typically fly at or close to maximum takeoff weights and so if QR decide to use higher standard weights 77W/A350 flights would have to leave behind payload.

Weighing hand luggage can be useful especially for performance critical flights or flights with known issues to verify assumptions are accurate as well as catch heavier bags and gate check them.