There is obviously no further reply as it is mandatory to have reversers in at least idle after touch down. An increase in the stopping distance is required if there is no reverser selection possible as this would be a non-normal condition.
The standard configuration after touch down, means reversers at least in idle, was used during the certification process to determine the stopping distance required.
Using more than idle reverse will substantially reduce the required braking efforts to maintain the same stopping distance. This will, however, involve another stress on the engines and reduce their life.
A 737 with a Vref of about 140kn (~54,000 kgs LW, flaps 30) will be able to vacate a runway after about 2500 meters in taxi speed even with no brakes, just reversers in idle.
Reverse thrust is anyway only very efficient above 100 kn, pilots with a feeling for the aircraft will notice the increasing force of the brakes when slowing down through about 80 kn.