WH, as Silver hawk stated the "rated" takeoff is carried out to reduce wear and tear on the engines, and also to reduce fuel consumption. The calculations are carried out using the information available regarding obstacles in the "obstacle accountability area" along the extended runway.
It is more than a little worrying that ALL pallets are not weighed. How much time/effort would it take to have a set of scales that the pallets are placed on at some point prior to loading and the weight recorded? That way you would know exactly the weight on board for M & B calcs, and from their positions you could work out the C of G.
Any reason why this isn't done?
Surely it would be economically viable even if it saved just one hull loss? How many hull losses have been attributable to incorrect masses being used?