Single disc brake front of SS bike?
Like the late great Sheldon Brown wrote in his article, I use the front brake alone most of the time.  But there are great reasons to have a rear brake in addition to the front brake:


Slippery pavement (wet weather, etc).  If the front wheel skids, the rider will almost always crash.  The rider can often recover from a rear-wheel skid, so if conditions are slippery, I try to use the rear brake and give myself plenty of time to stop.
Leverage.  When I have my hands on top of the brake hoods, I don't have as much leverage as when I'm grabbing the brake levers from the drops.  When I have to make a sudden emergency stop from the hoods, I can stop better when I use both brakes.
Braking while turning.  If the rider tries to use the front brake while turning sharply, that might cause the front wheel to skid.  Sometimes braking with both brakes is better in a sharp turn.  Also if the rear wheel skids, the rider might be able to recover, or the crash might be more gentle than a crash caused by a front-wheel skid.  I generally try to avoid braking while turning sharply, but sometimes I have no choice, if for instance I want to avoid hitting the child who ran onto the bike path without looking.
Equipment failure.  If there is some problem with the front wheel or the front brake, having a rear brake can be very handy.