I'm going to show my lack of cycling knowledge here, but I really don't understand what you're saying. Why wouldn't one just use the rear break and set their body, like when one rides a bus and has to stand?
Well, the front brake has ~70% of the stopping power (both on bikes and cars), so while that will work (assuming you don't skid in the back), it won't be the fastest. You can get quite a bit of stopping out of your front brake without going over, you just have to be precise.
You also have to keep in mind road bikes don't have disc brakes so precision is not part of road bike brakes. Those are most likely caliper brakes, which actually lock up fairly quickly when pressure is applied to the brakes.
Modern road bike brakes are plenty powerful enough with adequate modulation.
The real limiting factor on road bikes is traction.
Disc brakes on road bikes might give a distinctive advantage, even with greater weight. Disc brake rims need not have a braking surface and thus can be lighter having less angular momentum.
You've got a lot more ability to shift your weight on a bike than on a car.
The only time my rear wheel has left the ground while braking is when I've tried to do it intentionally. And I've made plenty of hard stops in thousands of miles of riding.
In addition to the other explanations about braking power, the reason why you have little breaking power is that most of your weight is over the front tire -- especially going downhill. Locking the back wheel up, means it just skids across the road.
It might be easy to go into a skid and lose control. I guess it depends on road surface, and maybe it's not an issue for the pros, but I nearly got into an accident once trying to suddenly stop using rear brakes while going a bit downhill.
