Jettesen
In your post you said "The paper work for weights, and loading is not done at the aircraft, but in the ground handlers offices, which could be 10 mins away from the aircraft. So 30 mins is not a lot of time to apply any changes to the Passenger loads or baggage."
If they are using standard figures, passengers weights is not a problem, one more passenger and luggage would make no difference to the aircraft W+B.
More a case of the aggravation.
As my Dad always said "better to be half hour early than 5 minutes late"