Flex / derate / assumed temp - call it what you will, but it DOES increase engine life and it DOES save real hard money. We had a memo in our airline reminding us to use the correct assumed temp reduction for every take off and the savings quoted were staggering.
Anyhoo - all those advocating full thrust for take off, you still have to do the performance calcs and if you make the same mistake made here and in Halifax with the 747 you are still going to rotate underspeed, still going to drag along the runway with the tail, but with alot more thrust. You will probably get airborne, but then again who knows.
Besides full thrust take offs can also have their own risk. Especially in a light aircraft, so maybe you are just swapping one risk for another, not reducing the overall risk level.