Hi Centaurus,
there is only ten feet difference between effect of reverse thrust engine-out RTO, brakes, spoilers and one thrust reverser and effect of reverse thrust all-engines RTO, brakes, spoilers and two reversers.
True on a dry runway, but the difference on a runway which has braking action reported as "Poor" is 530m (for A320).
Take OFF Flap + Ground Speed + Reverse selected is also the logic for Ground Spoilers and Auto Brakes on most aircraft.
RTO with REV selected on all engines is to keep things simple with just one procedure for all conditions.
There haven't been that many RTOs due engines fires which have sprayed fuel over the fuselage.