brake temperature
With autobrake 1-2-3 on a runway with good braking action reverse thrust above idle does not give you any advantage in relation to landing distance.
As soon as reverse is starting to assist in the deceleration, the pressure on the wheel brakes is being reduced, as autobrake is looking for a certain deceleration rate. So in normal circumstances reverse thrust is only easing the use of the brakes, which keeps your brake temperature nicely under control - however your landing distance will be exactly the same with or without T/R.
Things are different in autobrake max, max manual braking or runway conditions where anti skid is going to operate: 2nd or 3rd detent is going to give you a nice reduction in landing distance (if you instantly use it, as it was already said before...)