How often do you hear it – 'don't bother with the rear brake, because the wheel is hardly on the ground.' Something what is absolutely not true...
Of course the front brake is the primary stopping power on just about any motorcycle built since about 1950, but the rear brake isn't there just for show. If the wheel is indeed on the ground, the rear brake will offer extra retardant force and make the difference between stopping in time or not. Add a bit of rain to the situation and the rear brake becomes all the more important.
Then there's balancing a motorcycle at low speed. Keep jabbing at the front brake at anything under walking speed and you'll make a fine hash of any low speed manoeuvre. Every stop to a halt should conclude with the rear brake employed. In so many ways the rear caliper helps keep the motorcycle balanced and to ignore it is to throw away a large proportion of a motorcycle's ability.
Even combined braking systems don't override the need to use that pedal, as they work proportionally. Jam on the front and that will be the wheel to lock; the rear... Us both brakes together and you will stop faster without locking either.