A afdditional consideration is the initial cost of the aircraft. eg aircraft 'a' is sold at say 580,000lbs MTOW. To get it at say 600,000 costs more even if there is no other change except the performance curves in the AFM? Why, its called a derived benefit. You operate at higher weight for more profit and therefore the manufacturer gets some of that benefit...can be a very large sum per 1000lb of weight. The engine manufacturers are also in on this. An engine certified at say 45,000lbs thrust will be less than the same engine at 50k thrust? Only the program plug on the ECU are different and the AFM...so basically the manufacturers putting the icing on the cake eh? Even if by operating at higher weight / thrust you wear it out quicker and pay more in spares!! Wish I was in the game selling this!!!