Because of dramatically reduced rake/trail, the transfer of weight to the front during hard braking has dramatically different effects. On a cruiser, the rear wheel will still have traction during hard braking. On a sportbike, all of the traction will be on the front wheel during hard braking. 100%. This is why almost nobody uses the rear brake at all on the track. The only real use is to regulate rear-wheel spin while “stepping it in,” to stabilize the bike here and there, or to stop a power wheelie coming out of a corner.
I use the rear brake on the street for three thing: low speed maneuvers, holding the bike in place at a light/stop, and gradual braking. On my bikes, as soon as you’re braking hard on the front, the risk of locking the rear is too great to bother using it.