Originally Posted by
john_tullamarine
As indicated earlier, the problem lies with certification reliability. It would take lots of money to show that role equipment fuel dumping was OK
This...
Certification is intentionally done with conservatism - e.g. stopping and V1 reject speeds are based on not using the Thrust Reversers, because T/Rs can be MEL'd, and if the V1 reject is due to an engine failure, that T/R is useless.
Having positive margin in an emergency is always a good thing - while having negative margin can turn an emergency into a catastrophe.