Chuboy, our airline does standard pax weight surveys every couple of years and the standard weights used in the load control system are accurate enough for day to day use.
Where one flight might be able to take a couple of hundred kg more freight the next one might lose a couple of hundred kg, so overall the benefit is, in my humble opinion, likely to be not worth the considerable effort.
At the end of the day, if it is a safety issue we would see aeroplanes with weight and balance issues every day, which we don't. If we were seeing weight being left behind every day then management would be bleating loudly and whilst management tend to bleat loudly all the time it isn't about this issue.
I just can't see it happening. Weighing every punter and then using the data in the load control system adds complexity. complexity adds cost and cost is something most managers want to avoid. Not even to broach the fact that most, if not all, major load control systems could not accept a seat based weight for each punter, so it wouldn't happen for years if it did happen.