Just to plays devils advocate,
If something goes wrong that requires TOGA power, if it is below V1, you probably should have stopped anyway, and if you are past v1 well... you will have more power already being made so the power change will be smaller minimizing the chance of inducing upset.
Furthermore some flex/derates are so massive as to have an enourmous effect on min V1 (which is a function of Vmc and Vmcg) so becarefull about taking 2 derates plus a flex (in a 777 this can cut an engine thrust almost in half) and then cobbing the engines. You may get a nasty suprise as you head off the side of the runway....
In principal, it is quite true that there is nothing quite so useless as runway behind you. But once you get into the regime of flex power (reduced thrust) takeoffs you are leaving MILES of runway behind you even if the aircraft starts from the physical end of the runway... So that old axiom doesn't really survive in the modern age.
HOWEVER, blasting down the runway in your C150 you bet. Use every inch of the runway. Its just that in jets you really leave your runway behind you when you dial in a flex number, not where you physically start rolling from...
Cheers
Wino