Kinda depends on what kind of motorcycle i think. I rode a 69 Triumph Bonnie, hardtail bobber from Ohio to NC with just a rear brake and it was fine. On that bike you are sitting below the handlebars.
Presumably 'it was fine' because you never ran into a situation where you needed to stop as fast as possible. Even on that motorcycle, stopping with just a rear brake will give you significantly less than half the stopping force you'd have with front and rear brakes.