Flex/Assumed/Derated:
Longer take off rolls =
More chance to hit birds, FOD, blow a tire
Less runway to stop in a reject
Gives pilots less reaction time to an event
More V1 -VR Spread
Further down runway, closer to obsticles
Less obsticle clearance on climb out
Fierce debate among pilots? You bet...when managment wants to put put people at risk to save a buck, not surprised that the smart pilots step up and say something...